Adds support for the XLP on-chip PCIe controller. On XLP, the
on-chip devices(including the 4 PCIe links) appear in the PCIe
configuration space of the XLP as PCI devices.

The changes are to initialize and register the PCIe controller,
enable hardware byte swap in the PCIe IO and MEM space, and to
enable PCIe interrupts.
Signed-off-by: NGanesan Ramalingam <ganesanr@netlogicmicro.com>
Signed-off-by: NJayachandran C <jayachandranc@netlogicmicro.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3760/
Patchwork: https://patchwork.linux-mips.org/patch/4104/Signed-off-by: NRalf Baechle <ralf@linux-mips.org>