PCI/ASPM: Add L1 Substates definitions

Add and use #defines for L1 Substate register fields instead of hard-coding
the masks.  Also update comments to use names from the spec.  No functional
change intended.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Vidya Sagar <vidyas@nvidia.com>