ARM: dts: sun6i: Add simplefb node labels to reference at board level

Some boards, such as tablets, have regulators providing power to parts
of the display pipeline, like signal converters and LCD panels.

Add labels to the simplefb device nodes so that we can reference them
in the board dts files to add regulator supply properties.

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