dt-bindings: soc: Document "brcm,bcm2836-vchiq"

"brcm,bcm2836-vchiq" should be used on BCM2836 and BCM2837 to ensure
correct operation.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>