dt-bindings: display: sun4i: New compatibles for A20 tcons

Document new compatibles used to differentiate between timing
controllers on A20 (sun7i)

Signed-off-by: Andrey Lebedev <andrey@lebedev.lt>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200219180858.4806-5-andrey.lebedev@gmail.com