MIPS: Netlogic: Add support for built in DTB

Provide a config option to embed a device tree for XLP evaluation
boards. This DTB will be used if the firmware does not pass in a
device tree pointer.

Signed-off-by: Jayachandran C <jayachandranc@netlogicmicro.com>
Patchwork: http://patchwork.linux-mips.org/patch/4103/


Signed-off-by: John Crispin <blogic@openwrt.org>