Raspberry Pi 2 support, part 1: Interrupt controller

[Eric Anholt]

This is a little over half of the diff for the Raspberry Pi 2
platform.  A full series you can build and test can be found at:

https://github.com/anholt/linux/tree/bcm2836-irqchip

As you can see looking at that tree, it depends on the rpi-dt-clocks
series, which is still stalled waiting on the rpi-firmware series to
be merged (Lee said he pulled 2/3 of them back on June 5th, but they
haven't appeared in the tree).