MIPS: XLR, XLS: Add PCI support.

Adds pci/pci-xlr.c to support for XLR PCI/PCI-X interface and XLS PCIe
interface.
Update irq.c to ack PCI interrupts, use irq handler data to do the
PCI/PCIe bus ack.

Signed-off-by: Jayachandran C <jayachandranc@netlogicmicro.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/2337/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>