ARM: dts: explicitly mark skeleton.dtsi as deprecated

As noted in commit 3ebee5a2

 ("arm64: dts: kill skeleton.dtsi"),
there are a number of problems with skeleton.dtsi, and it would be
prefereable to remove it entirely. As there are a large number of
existing users, fixing these up will take a while.

This patch adds a note to arm's skeleton.dtsi noting that this is the
case, to make this more obvious and hopefully minimize new uptake of
skeleton.dtsi in the mean time.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Rob Herring <robh@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Olof Johansson <olof@lixom.net>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>