ARM: dts: sun4i: Remove skeleton and memory to avoid warnings

Using skeleton.dtsi will create a memory node that will generate a warning
in DTC. However, that node will be created by the bootloader, so we can
just remove it entirely in order to remove that warning.

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