ARM: dts: imx: Remove skeleton.dtsi

As explained by commit 9c0da3cc

 ("ARM: dts: explicitly mark
skeleton.dtsi as deprecated"), including skeleton.dtsi is deprecated.

This fixes the following warning with W=1:

Warning (unit_address_vs_reg): Node /memory has a reg or ranges property, but no unit name

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>