The A20 has two interconnected display pipelines, mirroring the A10.

Add all the device nodes for them, including the downstream HDMI
controller that we already support.
Signed-off-by: Jonathan Liu <net147@gmail.com>
[wens@csie.org: Squashed in HDMI and provided commit message]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>