ARM: dts: armv7-m: remove skeleton.dtsi include

Remove skeleton.dtsi from the common ARM Cortex-M dtsi. This will allow
us to remove skeleton.dtsi on a per platform basis and get rid of the
unit address warning on the memory nodes without getting duplicate memory
nodes.

See 3ebee5a2 ("arm64: dts: kill skeleton.dtsi") for additional
reasons not to use the skeleton.dtsi.

Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Vladimir Murzin <vladimir.murzin@arm.com>
Acked-by: Stefan Agner <stefan@agner.ch>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>