[bsp][c908]Added support for rt-smart and mmu Sv39/48 to QEMU xiaohui c908, and added mmu_demo for test.

[BadmintonMaster]

拉取/合并请求描述：(PR description)

[

为什么提交这份PR (why to submit this PR)

在QEMU xiaohui平台上，增加c908对rt-smart支持，并接入libcpu以支持mmu

你的解决方案是什么 (what is your solution)

1.修改原有libcpu中mmu框架，增加SV48/SV57支持，并提供测试mmu_demo
2.参考K230和Virt/C906实现方式，增加c908对rt-smart支持

请提供验证的bsp和config (provide the config and bsp)

• BSP:bsp\xuantie\xiaohui\c908

• .config:bsp\xuantie\xiaohui\c908.config

• action:

]

当前拉取/合并请求的状态 Intent for your PR

必须选择一项 Choose one (Mandatory):

• 本拉取/合并请求是一个草稿版本 This PR is for a code-review and is intended to get feedback
• 本拉取/合并请求是一个成熟版本 This PR is mature, and ready to be integrated into the repo

代码质量 Code Quality：

我在这个拉取/合并请求中已经考虑了 As part of this pull request, I've considered the following:

• 已经仔细查看过代码改动的对比 Already check the difference between PR and old code
• 代码风格正确，包括缩进空格，命名及其他风格 Style guide is adhered to, including spacing, naming and other styles
• 没有垃圾代码，代码尽量精简，不包含#if 0代码，不包含已经被注释了的代码 All redundant code is removed and cleaned up
• 所有变更均有原因及合理的，并且不会影响到其他软件组件代码或BSP All modifications are justified and not affect other components or BSP
• 对难懂代码均提供对应的注释 I've commented appropriately where code is tricky
• 代码是高质量的 Code in this PR is of high quality
• 已经使用clang-format 源码格式化工具确保格式符合RT-Thread代码规范 This PR has been formatted with clang-format and complies with RT-Thread code specification
• 如果是新增bsp, 已经添加ci检查到.github/ALL_BSP_COMPILE.json 详细请参考链接BSP自查

[github-actions]

👋 感谢您对 RT-Thread 的贡献！Thank you for your contribution to RT-Thread!

为确保代码符合 RT-Thread 的编码规范，请在你的仓库中执行以下步骤运行代码格式化工作流（如果格式化CI运行失败）。
To ensure your code complies with RT-Thread's coding style, please run the code formatting workflow by following the steps below (If the formatting of CI fails to run).

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🛠 操作步骤 | Steps

1. 前往 Actions 页面 | Go to the Actions page
   点击进入工作流 → | Click to open workflow →

2. 点击 Run workflow | Click Run workflow

• Use workflow from 保持默认分支（通常为 master）
  Keep the default branch (usually master) in Use workflow from
• 在 branch 输入框填写 PR 分支 c908_rttsmart_mmu
  Enter PR branch c908_rttsmart_mmu in the branch field
• 设置需排除的文件/目录（目录请以"/"结尾）
  Set files/directories to exclude (directories should end with "/")

3. 等待工作流完成 | Wait for the workflow to complete
   格式化后的代码将作为独立提交推送至你的分支。
   The formatting changes will be pushed to your branch as a separate commit.

完成后，提交将自动更新至 c908_rttsmart_mmu 分支，关联的 Pull Request 也会同步更新。
Once completed, commits will be pushed to the c908_rttsmart_mmu branch automatically, and the related Pull Request will be updated.

如有问题欢迎联系我们，再次感谢您的贡献！💐
If you have any questions, feel free to reach out. Thanks again for your contribution!

[github-actions]

📌 Code Review Assignment

🏷️ Tag: libcpu_riscv

Reviewers: @Yaochenger

Changed Files (Click to expand)

• libcpu/risc-v/common64/mmu.c
• libcpu/risc-v/common64/mmu.h
• libcpu/risc-v/t-head/c908/riscv_mmu.h

🏷️ Tag: workflow

Reviewers: @Rbb666 @kurisaW @supperthomas

Changed Files (Click to expand)

• .github/ALL_BSP_COMPILE.json

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📊 Current Review Status (Last Updated: 2026-08-31 15:47 CST)

• ⌛ @Rbb666 Pending Review
• ⌛ @Yaochenger Pending Review
• ⌛ @kurisaW Pending Review
• ⌛ @supperthomas Pending Review

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📝 Review Instructions

1. 维护者可以通过单击此处来刷新审查状态: 🔄 刷新状态
   Maintainers can refresh the review status by clicking here: 🔄 Refresh Status

2. 确认审核通过后评论 LGTM/lgtm
   Comment LGTM/lgtm after confirming approval

3. PR合并前需至少一位维护者确认
   PR must be confirmed by at least one maintainer before merging

ℹ️ 刷新CI状态操作需要具备仓库写入权限。
ℹ️ Refresh CI status operation requires repository Write permission.

[github-actions]

MemBrowse Memory Report

k230

• SRAM: .eh_frame +24 B, .text -16 B (+0.0%, 1,128,411 B / 268,300,288 B, total: 0% used)
  No memory changes detected for:
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