ARM: dts: orion5x: split linkstation lswtgl into common and device parts

In order to support more linkstation devices, common part of current
.dts file goes into .dtsi file. Some .dtsi start with "mvebu-" prefix
because other kirkwood based linkstation devices are similar, and
will be migrated to use these .dtsi some time later.
  - orion5x-linkstation.dtsi
  - mvebu-linkstation-fan.dtsi
  - mvebu-linkstation-gpio-simple.dtsi
while all rest part remains in device specific .dts file:
  - orion5x-linkstation-lswtgl.dts

Signed-off-by: Roger Shimizu <rogershimizu@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>