bcm2708: vchiq: Add Device Tree support

Turn vchiq into a driver and stop hardcoding resources.
Use devm_* functions in probe path to simplify cleanup.
A global variable is used to hold the register address. This is done
to keep this patch as small as possible.
Also make available on ARCH_BCM2835.
Based on work by Lubomir Rintel.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>