MIPS: Netlogic: Add XLP PIC irqdomain

Add a legacy irq domain for the XLP PIC interrupts. This will be used
when interrupts are assigned from the device tree. This change is required
after commit c5cdc67a

 "irqdomain: Remove temporary MIPS workaround code".

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Cc: linux-mips@linux-mips.org
Cc: Jayachandran C <jchandra@broadcom.com>
Patchwork: https://patchwork.linux-mips.org/patch/5597/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>