mfd: cros_ec: Allow building for ARM64

There are platforms using the ChromeOS embeded controller on ARM64 now,
so let's allow using this driver (without having to use COMPILE_TEST).

Signed-off-by: Brian Norris <briannorris@chromium.org>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>