Initialize gpio_keys driver at late_initcall level, to give it a chance to
work with GPIO expanders that might not be ready yet if we initialize the
driver at module_init time.

This is strictly a band-aid until there is a better way to specify
inter-device dependencies.
Signed-off-by: David Jander <david@protonic.nl>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>