drm/sun4i: frontend: Hook-in support for the A20

This adds the appropriate device-tree compatible for hooking frontend
support for the A20. Since the hardware is very similar to the A10, it
shares the same quirks (which were already introduced).

Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190118145133.21281-24-paul.kocialkowski@bootlin.com