ARM: gemini: delete <mach/gpio.h>

The Gemini has no need of a <mach/gpio.h> header as this is only
used by the machine-local gpio.c gpiochip driver, which is also
only using the irq_to_gpio() macro. Delete the file, move the single
macro into the driver and remove the NEED_MACH_GPIO_H flag.

Cc: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>