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

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