MIPS: lantiq: add support for xrx200 firmware depending on soc type

VR9 needs different firmware files for the various phy/soc revisions. Some
boards are ship with older and newer SoC revisions. To be able to boot a single
image on all versions we need to define both firmware files inside the
devicetree.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/8045/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>