Update the pci_cfg_save/restore routines to operate on bridge devices
(type 1 and type 2) as well as leaf devices (type 0). In particular,
this allows the existing PCI bus logic to save and restore capability
registers such as MSI and PCI-express work for bridge devices rather than
requiring that code to be duplicated in bridge drivers. It also means
that bridge drivers no longer need to save and restore basic registers
such as the PCI command register or BARs nor manage powerstates for the
bridge device.

While here, pci_setup_secbus() has been changed to initialize the 'sec'
and 'sub' fields in the 'secbus' structure instead of requiring the pcib
and pccbb drivers to do this in the NEW_PCIB + PCI_RES_BUS case.

Differential Revision: https://reviews.freebsd.org/D2240
Reviewed by: imp, jmg
MFC after: 2 weeks