irqchip: Add documentation for the bcm2836 interrupt controller

This is a new per-cpu root interrupt controller on the Raspberry Pi 2,
which will chain to the bcm2835 interrupt controller for peripheral
interrupts.

Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Cc: linux-rpi-kernel@lists.infradead.org
Cc: Lee Jones <lee@kernel.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1438902033-31477-4-git-send-email-eric@anholt.net


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>