ARM: dts: mps2: remove skeleton.dtsi include and fix unit address warnings

Removale of skeleton.dtsi allows us also to fix the following
warning from the dts compiler:
  Warning (unit_address_vs_reg): Node /memory has a reg or ranges
  property, but no unit name

by adding proper unit addresses to the memory nodes.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>