MIPS: Netlogic: XLP9XX PIC OF support

Support for adding legacy IRQ domain for XLP9XX. The node id of the
PIC has to be calulated differently for XLP9XX.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6286/