arm64: lib: use pair accessors for copy_*_user routines

The AArch64 instruction set contains load/store pair memory accessors,
so use these in our copy_*_user routines to transfer 16 bytes per
iteration.

Change-Id: I1adb007b4c8e09448e504f04a394340cd6fa22e8
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Patch-mainline: linux-arm-kernel @ 07/07/2015 @ 11:21
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>