ARM: mv78xx0: delete local <mach/gpio.h> header

This header is not used by anything, and since the arch does
not specify NEED_MACH_GPIO_H it is not included into
<linux/gpio.h> either. Delete it.

Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>