arm64: Change memcpy in kernel to use the copy template file

This converts the memcpy.S to use the copy template file. The copy
template file was based originally on the memcpy.S

Signed-off-by: Feng Kan <fkan@apm.com>
Signed-off-by: Balamurugan Shanmugam <bshanmugam@apm.com>
[catalin.marinas@arm.com: removed tmp3(w) .req statements as they are not used]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>