PCI: Add Enhanced Allocation register entries

Add registers defined in PCI-SIG's Enhanced allocation ECN.

[bhelgaas: s/WRITEABLE/WRITABLE]
Signed-off-by: Sean O. Stalley <sean.stalley@intel.com>
[david.daney@cavium.com: Added more definitions for PCI_EA_BEI_*]
Signed-off-by: Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>