drm/sun4i: Add support for A20 display pipeline components

The A20 display pipeline has 2 frontends, 2 backends, and 2 TCONs.
This patch adds support (or a compatible string in the frontend's
case) for these components.

The TCONs support directly outputting to CPU/RGB/LVDS LCD panels,
or it can output to HDMI via an on-chip HDMI controller, or
CVBS/YPbPr/VGA signals via on-chip TV encoders. These additional
encoders are not covered in this patch.

Signed-off-by: Jonathan Liu <net147@gmail.com>
[wens@csie.org: Expand commit message]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171017121807.2994-6-wens@csie.org