riscv: io.h: Fix signatures of reads/writes functions

Change type of address parameter from int* to volatile void* for
{read,write}s{b,w,l}() functions and add const qualifier for reads. This
is done to keep function signatures in sync with asm-generic/io.h and
other platforms.

Signed-off-by: Igor Prusov <ivprusov@salutedevices.com>