dt-bindings: simplefb-sunxi: add pipelines for DE2

As we're going to add simplefb support for Allwinner SoCs with DE2, add
suitable pipeline strings in the device tree binding.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>