arm64: 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.

Change-Id: I0fa7b172b1a5caa5e0897c47a53636b20cd6df2c
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>