ARM: tegra: Remove usage of deprecated skeleton.dtsi

Remove the usage of skeleton.dtsi because it was deprecated since commit
9c0da3cc ("ARM: dts: explicitly mark skeleton.dtsi as deprecated").
It also allows later to fix DTC warnings for missing unit name in
/memory nodes.

Compiled DTBs are the same as before this commit.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>