powerpc: Don't set HAVE_EFFICIENT_UNALIGNED_ACCESS on little endian builds

POWER7 takes alignment exceptions on some unaligned addresses, so
disable HAVE_EFFICIENT_UNALIGNED_ACCESS. This fixes an early boot
issue in the printk code.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>