MIPS: PCI: Split pci.c into pci.c & pci-legacy.c

MIPS: PCI: Split pci.c into pci.c & pci-legacy.c

Split out the parts of pci.c that are used by existing systems with
MIPS-style PCI drivers but that will not be used by systems with more
generic PCI drivers such as pcie-xilinx. This is done in preparation for
allowing configurations where the code moved to pci-legacy.c is not
built.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: Paul Burton <paul.burton@imgtec.com>
Patchwork: https://patchwork.linux-mips.org/patch/14344/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>