drm/sun4i: tcon: Add support for R40 TCON

R40 TV TCON is basically the same as on A83T. However, it needs special
handling, because it has to set up TCON TOP muxes at runtime.

Reviewed-by: Chen-Yu Tsai <wens@csie.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/20180710203511.18454-14-jernej.skrabec@siol.net