[Feature] risc-v: 统一 PLIC 支持 | Unify the PLIC

[polarvid]

Describe problem solved by the proposed feature

以下三个文件分别实现了 PLIC 驱动，可以统一到一个实现中。

• https://github.com/RT-Thread/rt-thread/blob/master/libcpu/risc-v/t-head/c906/plic.c
• https://github.com/RT-Thread/rt-thread/blob/master/libcpu/risc-v/t-head/c908/plic.c
• https://github.com/RT-Thread/rt-thread/blob/master/libcpu/risc-v/virt64/plic.c

Describe your preferred solution

No response

Describe possible alternatives

No response