PCI: xilinx: Allow build on MIPS platforms

Allow the xilinx-pcie driver to be built on MIPS platforms which make use
of generic PCI drivers rather than legacy MIPS-specific interfaces.  This
is used on the MIPS Boston development board.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Bharat Kumar Gogada <bharatku@xilinx.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Ravikiran Gummaluri <rgummal@xilinx.com>