ARM: dts: sun6i: Change framebuffer node names to avoid warnings

The simple-framebuffer nodes have a unit address, but no reg property which
generates a warning when compiling it with DTC.

Change the simple-framebuffer node names so that there is no warnings on
this anymore.

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