PCI: generic: Add arm64 support

Make pci-host-generic driver (kernel option PCI_HOST_GENERIC) available on
arm64.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
(cherry picked from commit aa4a5c0d2d7e3c30f9df033ea0367f148bb369f6)
Signed-off-by: Alex Shi <alex.shi@linaro.org>