pci: Add ability to re-enumerate PCI buses

pci: Add ability to re-enumerate PCI buses

Add a new 'pci enum' command which re-enumerates the PCI buses.  This
command is enabled via the CONFIG_CMD_PCI_ENUM define and can be useful
in boards with FPGAs connected via PCI/PCIe, boards that support PCI
hot-plugging, or during PCI debug.

Also enable the 'pci enum' command for X-ES's Freescale-based boards.

Signed-off-by: John Schmoller <[email protected]>
Signed-off-by: Peter Tyser <[email protected]>
Acked-by: Kumar Gala <[email protected]>
Acked-by: Wolfgang Denk <[email protected]>