PCI: Add pcie_ptm_enabled()

Add a predicate that returns if PCIe PTM (Precision Time Measurement)
is enabled.

It will only return true if it's enabled in all the ports in the path
from the device to the root.

Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>