dt/bindings: arm-boards: Remove skeleton.dtsi inclusion from example

As of commit 9c0da3cc ("ARM: dts: explicitly mark skeleton.dtsi
as deprecated"), including skeleton.dtsi is deprecated.
Hence remove it from the example.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Rob Herring <robh@kernel.org>