csky: optimize memcpy_{from,to}io() and memset_io()

Optimize memcpy_{from,to}io() and memset_io() by transferring in
64 bit as much as possible with minimized barrier usage. This
simplest optimization brings faster throughput compare to current
byte-by-byte read and write with barrier in the loop. Code's
skeleton is taken from the powerpc & arm64.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Signed-off-by: Guo Ren <guoren@kernel.org>