PCI: xgene: Add APM X-Gene PCIe driver

Add the AppliedMicro X-Gene SOC PCIe host controller driver.  The X-Gene
PCIe controller supports up to 8 lanes and GEN3 speed.  The X-Gene SOC
supports up to 5 PCIe ports.

[bhelgaas: folded in MAINTAINERS and bindings updates]
Tested-by: Ming Lei <ming.lei@canonical.com>
Tested-by: Dann Frazier <dann.frazier@canonical.com>
Signed-off-by: Tanmay Inamdar <tinamdar@apm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Liviu Dudau <Liviu.Dudau@arm.com> (driver)