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>

arch/arm/boot/dts/skeleton.dtsi

 /*
+ * This file is deprecated, and will be removed once existing users have been
+ * updated. New dts{,i} files should *not* include skeleton.dtsi, and should
+ * instead explicitly provide the below nodes only as required.
+ *
  * Skeleton device tree; the bare minimum needed to boot; just include and
  * add a compatible value.  The bootloader will typically populate the memory
  * node.