risc-v: Move CSR register manipulation macros to csr.h

[no1wudi]

Summary

This patch allow public arch level code use them, make it possible to access system register in common code, such as percpu.

Impact

RISC-V Only

Testing

GitHub CI and local machine

[nuttxpr]

[Experimental Bot, please feedback here]

No, this PR does not fully meet the NuttX requirements. The summary is insufficiently detailed. It doesn't explain why making system registers accessible in common code is necessary (what problem does it solve?). It vaguely mentions percpu, but doesn't explain how the change works or what specifically changes. It's missing links to related issues.

The Impact section lacks detail. While it specifies RISC-V, it doesn't describe the changes to the architecture/boards/drivers. It's missing information about documentation, security, compatibility, and "anything else to consider."

The Testing section is also inadequate. It mentions GitHub CI and a local machine, but provides no details about the host or target environments. Critically, it lacks "before" and "after" testing logs, making it impossible to verify the change's effectiveness.