Add interrupt support for GPIO pins on OMAP4 and AM335x.

This enables the use of GPIO pins as interrupt sources for kernel devices
directly attached to gpiobus (userland notification will be added soon).

The use of gpio interrupts for other kernel devices will be possible when
intrng is complete.

All GPIO pins can be set to trigger on:

- active-low;
- active-high;
- rising edge;
- falling edge.

Tested on:	Beaglebone-black