microblaze: support gpio_to_irq()

gpiolib returns -ENXIO if struct gpio_chip::to_irq isn't set, so it's
safe to always call.

Based on PPC patch
"powerpc/gpio: support gpio_to_irq()"
78331aded913d76c7ff996a1a0747d54b057460b

Signed-off-by: Michal Simek <monstr@monstr.eu>
1 file changed