arm64: dts: allwinner: a64: Add cross links for the mixers

Unlike what the binding for multiple pipeline documents, the A64 doesn't
have the cross links between the TCON and the mixers.

Let's add them.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>