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: NJeff Garzik <jeff@garzik.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>
Acked-by: NJeff Garzik <jeff@garzik.org>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

arch/arm/mach-lpc32xx/include/mach/gpio.h

+/* empty */

arch/arm/mach-tegra/include/mach/gpio.h

+/* empty */

arch/arm/mach-u300/include/mach/gpio.h

+/* empty */