ARM: lpc32xx: 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.

Acked-by: Roland Stigge <stigge@antcom.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>