ARM: 7135/1: ep93xx: bring back missing <mach/gpio.h>

Change bd5f12a2 (ARM: 7042/3: mach-ep93xx: break out GPIO driver specifics)
accidentally removed the ep93xx <mach/gpio.h> instead of making it an empty
file. This causes compilation to fail:

In file included from include/linux/gpio.h:18:0,
                  from drivers/gpio/gpiolib.c:10:
linux/arch/arm/include/asm/gpio.h:5:23: fatal error: mach/gpio.h: No such file or directory
compilation terminated.
make[2]: *** [drivers/gpio/gpiolib.o] Error 1

Fix this by adding the file back.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ryan Mallon <rmallon@gmail.com>
Signed-off-by: Mika Westerberg <mika.westerberg@iki.fi>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>