staging: vchiq_arm: Make DT firmware node mandatory

In Linux Mainline there wasn't a chance to boot the RPi without DT.
So we can make the firmware node mandatory.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>