riscv: Avoid io read/write cause wrong result

io read/write may cause wrong result because they may read/write data
from/to register instead of memory. Add 'volatile' to avoid it.

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