PCI: cadence: Add Power Management ops for host and EP

PCI: cadence: Add Power Management ops for host and EP

These PM ops will enable/disable the optional PHYs if present.  The
AXI link-down register in the host driver is now cleared in
cdns_pci_map_bus() since the link-down bit will be set if the PHY has
been disabled. It is not cleared when enabling the PHY, since the
link will not yet be up (e.g. when an EP controller is connected
back-to-back to the host controller and its PHY is still disabled).

Link: http://lkml.kernel.org/r/1529915453-4633-5-git-send-email-adouglas@cadence.com
Signed-off-by: Alan Douglas <adouglas@cadence.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>