ARM: dts: sun4i: 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.

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