ARM64: dts: meson-gx: Add Graphic Controller nodes

Add Video Processing Unit and CVBS Output nodes, and enable CVBS on selected
boards.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>