BCM270X: Add an overlay for enabling the vc4 driver in firmware-KMS mode.

This gets us normal 3D support on top of the existing firmware display
stack.  There's no real modesetting support, no async pageflips
(hurting performance), etc., but it means that the desktop can at
least run until we get full native modesetting.

Signed-off-by: Eric Anholt <eric@anholt.net>