ARM: optimize memset_io()/memcpy_fromio()/memcpy_toio()

If we are building for a LE platform, and we haven't overriden the
MMIO ops, then we can optimize the mem*io operations using the
standard string functions.

Acked-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>