ARM: dts: sun6i: Add second display pipeline device nodes

The Allwinner A31/A31s SoCs have 2 display pipelines, as in 2 display
frontends, backends, and tcons each. The relationship between the
backends and tcons are 1:1, but the frontends can feed either backend.

Add device nodes and of graph nodes describing this relationship.

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