PCI: xilinx: Convert to use generic config accessors

Convert the Xilinx host PCI driver to use the generic config access
functions.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Michal Simek <michal.simek@xilinx.com>
CC: "Sören Brinkmann" <soren.brinkmann@xilinx.com>
CC: linux-arm-kernel@lists.infradead.org