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 <jschmoller@xes-inc.com>
Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Wolfgang Denk <wd@denx.de>