riscv: define find_{first,next}_zero_bit in asm/bitops.h

These seem to be missing, and trying to build fastboot cmd without
them is causing errors due to these being missing.

Signed-off-by: Maxim Kochetkov <fido_max@inbox.ru>
Tested-by: E Shattow <lucent@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>