arch/arm/am67: Add eCAP APWM lower-half driver

[gemstonedevteam]

Summary

• Adds a PWM lower-half driver for the AM67 eCAP modules in APWM mode, where
  the module is used as a single-channel PWM generator rather than a capture
  unit.
• Period is programmed in CAP1 and the compare value in CAP2, with the
  counter free-running from the eCAP functional clock at 125 MHz.
• start() reprograms the time base only when the frequency changes, so a
  duty-only update does not clear TSCNTSTP or zero TSCTR on a running
  counter.
• t3-gem-o1 registers eCAP1 as /dev/ecap1 and eCAP2 as /dev/ecap2. Output
  pad selection is left to the board; the candidate pads collide with I2C0 on
  this design.

Impact

• Is new feature added? YES — eCAP APWM support for AM67.
• Impact on user? NO — new driver, opt-in by Kconfig.
• Impact on build? NO.
• Impact on hardware? YES — arch/arm/am67 and board t3-gem-o1.
• Impact on documentation? YES — provided in this PR (second commit).
• Impact on security? NO.
• Impact on compatibility? NO — additive.
• Anything else? Depends on PRs 3–5.

Testing

Build Host(s): Pardus GNU/Linux 25 (Debian 13 base), x86_64, host gcc 14
Toolchain:     arm-none-eabi-gcc 14.2.1 20241119 (15:14.2.rel1-1)
Target:        arm / TI AM67 (J722S) Cortex-R5F, board t3-gem-o1:nsh
nuttx-apps:    5a7ab4200
Loaded on the main-domain R5F by Linux remoteproc; console on UART-MAIN1.

Testing logs before change: no eCAP support on this arch.

The version string in the logs is from the tested build; the commits were
GPG-signed afterwards, which rewrites hashes. The code is unchanged.

Testing logs after change:

make distclean && ./tools/configure.sh t3-gem-o1:nsh && make -j
  -> nuttx ELF 543332 bytes, 0 compiler warnings

Console:
nsh> pwm -p /dev/ecap1 -f 1000 -d 50 -t 1
pwm_main: starting output with frequency: 1000 channel: 1 duty: 00007fff channel: 2 duty: 00007fff
pwm_main: stopping output
nsh> pwm -p /dev/ecap2 -f 1000 -d 50 -t 1
pwm_main: starting output with frequency: 1000 channel: 1 duty: 00007fff channel: 2 duty: 00007fff
pwm_main: stopping output
nsh> pwm -p /dev/ecap1 -f 1000 -d 25 -t 2
pwm_main: starting output with frequency: 1000 channel: 1 duty: 00003fff channel: 2 duty: 00007fff
pwm_main: stopping output
nsh> pwm -p /dev/ecap1 -f 1000 -d 75 -t 2
pwm_main: starting output with frequency: 1000 channel: 1 duty: 0000bfff channel: 2 duty: 00007fff
pwm_main: stopping output
nsh>

Test plan result:
console_alive            PASS  prompt responsive
version                  PASS  NuttX version 0.0.0 aaf0c2583d Sep 10 2026 13:49:06 t3-gem-o1:nsh
procfs                   PASS  procfs readable
tasks                    PASS  2 task lines
dev_nodes                PASS  all expected nodes present
pwm_ecap1                PASS  /dev/ecap1 1 kHz / 50% for 1 s
pwm_ecap2                PASS  /dev/ecap2 1 kHz / 50% for 1 s
ecap_duty_only           PASS  25% then 75% at 1 kHz accepted

PASS=8

Measured time base, captured 2026-09-11, read back from the A53 over
/dev/mem while NuttX drives the output.

NuttX:  nsh> pwm -p /dev/ecap1 -f 1000 -d 50 -t 25 &

A53, while that is running (eCAP registers are 32-bit natively):

  /dev/ecap1 @ 0x23110000   APRD = 124999, ACMP = 62498
  /dev/ecap2 @ 0x23120000   APRD = 124999, ACMP = 62498

  frequency = 125 MHz / (124999 + 1) = 1000.00 Hz
  duty      = 62498 / 125000         = 49.998 %

  TSCTR sampled twice 0.4 s apart: 12959 -> 109982 (ecap1) and
  67938 -> 93142 (ecap2), so the counter is free-running.

This is also the direct evidence for the duty-only path above: the eCAP
clock is FICLK/2 of the EPWM's, and the 125000-tick period confirms the
125 MHz figure the driver assumes.

PR verification Self-Check

• This PR introduces only one functional change.
• I have updated all required description fields above.
• My PR adheres to Contributing Guidelines and Documentation.
• My PR is still work in progress (not ready for review).
• My PR is ready for review and can be safely merged into a codebase.

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb