ARM: mark empty gpio.h files empty

It is generally a better idea to make intentionally empty files
contain the human-readable /* empty */ comment, also it makes
the files play nice with "make distclean".

Reported-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>