ARM: dts: sun8i: r40: Enable HDMI output on BananaPi M2 Ultra

Since HDMI can be considered as main output, most capable mixer is
connected to it (mixer0).

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>