bcm2708: fix gpio_to_irq() name clash

<mach/gpio.h> has gpio_to_irq() defined as a macro. the macro is
obviously intended as the direct implementation of that
functionality. unfortunately the gpio subsystem offers a public
function of the same name through <linux/gpio.h>. one has to be very
careful to include <mach/gpio.h> before <linux/gpio.h> - otherwise the
code will compile but only work by chance. board code will certainly
not work - the gpio driver is simply not loaded at that time.

fix the clash by renaming the offending macros from <mach/gpio.h>,
together with their uses.