arm64: dts: meson-gx: add dmcbus and canvas nodes.

DMC is a small memory region with various registers,
including the ones needed for the canvas module.

Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Maxime Jourdan <mjourdan@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>