Add basic interrupt management code to gpiobus and ofw_gpiobus.

This is the general support to allow the use of GPIO pins as interrupt
sources for direct gpiobus children.

The use of GPIO pins as generic interrupt sources (for an ethernet driver
for example) will only be possible when arm/intrng is complete. Then, most
of this code will need to be rewritten, but it works for now, is better
than what we have and will allow further developments.

Tested on: ar71xx (RSPRO), am335x (BBB), bcm2835 (Raspberry pi)
Differential Revision: https://reviews.freebsd.org/D999
Reviewed by: rpaulo