ARM: kconfig: select HAVE_EFFICIENT_UNALIGNED_ACCESS for CPUv6+ && MMU

Modern ARM CPUs can perform efficient unaligned memory accesses in
hardware and this feature is relied up on by code such as the dcache
word-at-a-time name hashing.

Change-Id: I41559fc9255229e2601f4bef28aa8d48246156b7
Signed-off-by: Will Deacon <will.deacon@arm.com>