BCM270x: Switch to firmware driver

defconfig: enable BCM2835_MBOX, RASPBERRYPI_FIRMWARE and BCM_VCIO.
Add firmware node and change mailbox node in Device Tree.
Add/update platform file for firmware and mailbox.
Strip bcm2708-vcio of everything except the legacy API and hook it
up with the firmware driver.

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