examples/rng90: add Microchip RNG90 example application

[FelipeMdeO]

Summary

This PR adds a new RNG90 example application in apps.

Changes included:

• New RNG90 example source: examples/rng90/rng90_main.c
• Build integration files for Make and CMake:
  • examples/rng90/Makefile
  • examples/rng90/Make.defs
  • examples/rng90/CMakeLists.txt
• Kconfig integration:
  • examples/rng90/Kconfig

The example opens the RNG90 character device, reads random bytes, and prints
hex output. It supports optional command-line arguments for device path and
requested byte count.

How to enable via Kconfig:

• In nuttx (driver side), enable: Device Drivers -> Cryptographic Device Drivers -> RNG90 (CONFIG_DEV_RNG90=y)
• In apps, enable: Application Configuration -> Examples -> Microchip RNG90 TRNG example (CONFIG_EXAMPLES_RNG90=y)
• Optional app settings:
  • CONFIG_EXAMPLES_RNG90_PROGNAME
  • CONFIG_EXAMPLES_RNG90_DEVPATH
  • CONFIG_EXAMPLES_RNG90_PRIORITY
  • CONFIG_EXAMPLES_RNG90_STACKSIZE

Impact

• Adds a new user-space example under apps/examples.
• No changes to existing examples behavior.
• Requires RNG90 driver support in nuttx (CONFIG_DEV_RNG90) to run.

Testing

Host:

• macOS

Target:

• esp32c3-devkit
• RNG90 connected and enabled
• Serial settings: 115200 8N1

Build/flash used for validation:

• make -j4
• make flash ESPTOOL_PORT=

Hardware validation transcript:

nsh> uname -a
NuttX 12.6.0-RC1 4423760152-dirty Jun  7 2026 23:00:02 risc-v esp32c3-devkit

nsh> ls /dev/rng0
 /dev/rng0

nsh> rng90
RNG90 example: device /dev/rng0
Read 32 random byte(s):
4f fc 96 27 91 99 95 32 90 b4 46 3c 3d a0 4c e9
ce 86 6d 41 7e a4 ab 02 4f 3a fe 34 ef c8 d0 15

nsh> rng90 /dev/rng0 16
RNG90 example: device /dev/rng0
Read 16 random byte(s):
82 6e 45 0b 1c f1 df d3 75 32 19 32 eb a5 e6 cc

nsh> rng90 /dev/rng0 64
Invalid count 64, clamping to 32
RNG90 example: device /dev/rng0
Read 32 random byte(s):
8d 8d fb 91 07 01 41 e7 54 e4 8b 77 9a 1d e9 2b
83 55 18 b9 79 0b 26 1c 52 07 56 77 f7 34 aa 77

[cederom]

Thank you @FelipeMdeO :-)