dt-bindings: display: sunxi-drm: Add TCON TOP description

TCON TOP main purpose is to configure whole display pipeline. It
determines relationships between mixers and TCONs, selects source TCON
for HDMI, muxes LCD and TV encoder GPIO output, selects TV encoder
clock source and contains additional TV TCON and DSI gates.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180625120304.7543-5-jernej.skrabec@siol.net