riscv: do not reimplement generic io functions

RISC-V U-Boot reimplements the generic io functions from
asm-generic/io.h. Remove the redundant implementation and include the
generic io.h instead.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>