PCI: dra7xx: Add PM support

Add PM support to pci-dra7xx so PCI clocks can be disabled during suspend
and enabled during resume without affecting PCI functionality.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jingoo Han <jingoohan1@gmail.com>