This enables us to have a default device tree per SoC family to be built
into the kernel. The default device tree for XLP3xx has been added as part
of this change. Later this can be used to provide support default boards
for XLP2xx and XLP9xx SoCs.

Kconfig options are provided for each default device tree so that just the
needed ones can be selected to be built into the kernel.
Signed-off-by: NJayachandran C <jchandra@broadcom.com>
Patchwork: http://patchwork.linux-mips.org/patch/5023/Acked-by: NJohn Crispin <blogic@openwrt.org>