ARM: dts: sun7i: Add device nodes for display pipelines

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>