drm/sun4i: Add LVDS support

The TCON supports the LVDS interface to output to a panel or a bridge.
Let's add support for it.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/7fbb85f33ee1d5009fde4f0d7d236e11ca58b114.1513854122.git-series.maxime.ripard@free-electrons.com