ARM: dts: sun8i: r40: Add mixer ids to TCON TOP

sun4i-drm DT binding, second paragraph of the first section says:

For all connections between components up to the TCONs in the display
pipeline, when there are multiple components of the same type at the
same depth, the local endpoint ID must be the same as the remote
component's index.

Add mixer ids in R40 DT as mandated by DT binding.

Fixes: 05a43a26 ("ARM: dts: sun8i: r40: Add HDMI pipeline")
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>