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.

This patch selects HAVE_EFFICIENT_UNALIGNED_ACCESS for these cores and
reworks the kconfig select logic for DCACHE_WORD_ACCESS to use the new
symbol.

Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
Signed-off-by: zwliew <zhaoweiliew@gmail.com>