m68k: Fall back to __gpio_to_irq() for non-arch GPIOs

gpiolib provides __gpio_to_irq() to map gpiolib gpios to interrupts - hook
that up on m68k.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>