gpiolib: Don't prevent IRQ usage of output GPIOs

Upstream Linux deems using output GPIOs to generate IRQs as a bogus
use case, even though the BCM2835 GPIO controller is capable of doing
so. A number of users would like to make use of this facility, so
disable the checks.

See: https://github.com/raspberrypi/linux/issues/2527



Signed-off-by: Phil Elwell <phil@raspberrypi.org>