MIPS: PCI: Support generic drivers

MIPS: PCI: Support generic drivers

Introduce support for PCI drivers using only functionality provided
generically by the PCI subsystem, by adding the minimum arch-provided
functions required.

The driver this has been developed for & tested with the xilinx-pcie on
a MIPS Boston development board.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14346/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>