ARM: dts: armada-375: Remove skeleton.dtsi

The skeleton.dtsi file was removed in ARM64 for different reasons as
explained in commit ("3ebee5a2

 arm64: dts: kill skeleton.dtsi").

These also applies to ARM and it will also allow to get rid of the
following DTC warnings in the future:

"Node /memory has a reg or ranges property, but no unit name"

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>