drm: sun4i: add support for H6 TCON TOP

The TCON TOP on Allwinner H6 SoC is a cut down version of the R40 TCON
TOP, which dropped TCON_TV1 and DSI (which do not exist on H6).

Add support for it.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181104182705.18047-27-jernej.skrabec@siol.net