ARM: dts: kirkwood: split lswxl dts to linkstation lswsxl and lswxl

LS-WXL/WSXL are both kirkwood-6281 based 2-Bay NAS devices, which share
many MPP pins. However they are slightly different:
  - There're two red LED indicator on LS-WXL to show when HDD fails,
    but there's no such on LS-WSXL.
  - There's 4-level speed adjustable FAN on LS-WXL, but not LS-WSXL.

So after the split, common part goes into .dtsi file:
  - kirkwood-linkstation.dtsi
  - kirkwood-linkstation-duo-6281.dtsi
while all rest part goes into device specific .dts file:
  - kirkwood-linkstation-lswsxl.dts
  - kirkwood-linkstation-lswxl.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>