bcm2835-aux: Add aux interrupt controller

The AUX block has a shared interrupt line with a register indicating
which devices have active IRQs. Expose this as a nested interrupt
controller to avoid sharing problems.

See: https://github.com/raspberrypi/linux/issues/1484
     https://github.com/raspberrypi/linux/issues/1573



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