ARM: dts: sun9i: Convert a80 optimus board dts to label referencing

The preferred method of referencing nodes from the dtsi is to use
labels, instead of copying the complete tree.

This patch converts sun9i-a80-optimus.dts to use label references.

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