riscv: Clean up asm/io.h

Clean up asm/io.h by
- removing commented code
- removing outdated comments
- removing unused definitions (for mem_isa, mem_pci)

This massively improves the readability of the file.

Suggested by commits:
7ab2e47d ("arm: Clean up asm/io.h")
909d0399 ("ARM: asm/io.h: kill off confusing #ifdef __mem_pci block")

Signed-off-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>