PCI: xgene: Convert to use generic config accessors

Convert the xgene 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: Tanmay Inamdar <tinamdar@apm.com>
CC: linux-arm-kernel@lists.infradead.org