drm: sun4i: Add composite output

Some Allwinner SoCs have an IP called the TV encoder that is used to output
composite and VGA signals. In such a case, we need to use the second TCON
channel.

Add support for that TV encoder.

Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>