bcm2708: remove NEED_MACH_GPIO_H

A recent commit of mine, f04ff75d39215e8b0ecd20c027abda45bf741213, removes the macro gpio_to_irq() from mach/gpio.h. Digging through the whole mess of the various gpio.h incarnations, I found that it's probably best to rename that file to gpio_irq.h, and to remove NEED_MACH_GPIO_H from the architecture.

See http://lists.infradead.org/pipermail/linux-arm-kernel/2012-September/118375.html which led me to the point that this has to go.