ARM: dts: orion5x: Add description for Netgear WNR854T

ARM: dts: orion5x: Add description for Netgear WNR854T

This is a router based on the mv88f5181 chipset.

http://www.netgear.com/support/product/WNR854T.aspx
http://wiki.openwrt.org/toh/netgear/wnr854t

[gregory.clement@free-electrons.com:
- extract dt part from "arm: orion5x: Add DT-based support for Netgear
  WNR854T"
- squashed "arm: orion5x: Alias uart0 to serial0 for all orion5x" into
this commit and move serial0 alias from dtsi to dts]
Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>