drm/sun4i: tcon: Add support for demuxing TCON output on A31

On systems with 2 TCONs such as the A31, it is possible to demux the
output of the TCONs to one encoder.

Add support for this for the A31.

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/20171010032008.682-3-wens@csie.org