overlays: Add audio parameter to vc4-kms-v3d

The audio parameter to the vc4-kms-v3d overlay allows audio support
to be disabled (it defaults to on) by adding "audio=off" to the
dtoverlay parameters.

See: https://github.com/raspberrypi/linux/issues/2489



Signed-off-by: Phil Elwell <phil@raspberrypi.org>