ARM: make mach/gpio.h headers optional

Most platforms don't need mach/gpio.h and it prevents multi-platform
kernel images. Add CONFIG_NEED_MACH_GPIO_H and make platforns select it
if they need gpio.h. This is platforms that define __GPIOLIB_COMPLEX
or have lots of implicit includes pulled in by mach/gpio.h.

at91 and omap have gpio clean-up pending and can drop
CONFIG_NEED_MACH_GPIO_H once that is in.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: Russell King <linux@arm.linux.org.uk>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Acked-by: Linus Walleij <linus.walleij@linaro.org>