gpio/omap: implement irq mask/disable with proper semantic.

When a gpio interrupt is masked, the gpio event will still be latched in
the interrupt status register so when you unmask it later you may get an
interrupt straight away. However, if the interrupt is disabled then gpio
events occurring will not be latched/stored.

Signed-off-by: Andreas Fenkart <andreas.fenkart@streamunlimited.com>