riscv: add generic link for <asm/atomic.h>

Add a link from <asm/atomic.h> to the generic one to allow
things like ubifs to be built. This can be extended with
riscv AMO ops at a later date.

Signed-off-by: Ben Dooks <ben.dooks@sifive.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>