MIPS: Netlogic: Add default DTB for XLP9XX SoC

Add a default device tree fie for XLP9XX boards, and add code to use
this device tree if no DTB is passed to the kernel.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6287/