arm64: dts: kill skeleton.dtsi

While skeleton.dtsi was initially conceived as a simple way to bootstrap
writing a dts, it has proven to be problematic:

* The #address-cells and #size-cells values used in skeleton.dtsi may
  not match what a user wants (e.g. when they need to describe a range
  larger than 4GB).

* For dts files where memory nodes have unit-addresses, it adds a
  redundant /memory node, for which the reg entry may not be
  appropriately sized (e.g. where #size-cells has been overridden).

* For dts files which assume that a bootloader will fill in the memory
  node(s), no node is present in the dts (and hence there is no attached
  comment), making it hard to distinguish these cases from bad dts
  files, and masking any warnings dtc may produce w.r.t. missing nodes.

* The default empty /chosen and /aliases are somewhat useless, and it
  would be preferable for dts to fill these in (e.g. for
  /aliases/serial0 and /chosen/stdout-path).

This patch removes skeleton.dtsi from arm64. There are currently no
users, so we can remove it before any appear.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Rob Herring <rob.herring@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Olof Johansson <olof@lixom.net>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>