pci: Extend 'pci' command with bus option '*'

Allow to call 'pci' and 'pci regions' commands with bus option '*' which
cause pci to process all buses.

PCIe is point-to-point HW and so on each bus is maximally one physical
device. Therefore for PCIe it is common to have multiple buses.

This change allows to easily print all available PCIe devices in system.

Make '*' as default option when no bus argument is specified.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>