ARM: 7056/1: plat-nomadik: kill off <plat/gpio.h>

After recent consolidations the Nomadik <plat/gpio.h> is entirely
superfluous, so get rid of it.

Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>