pci: Extend 'pci regions' command with bus number

'pci regions' currently prints only region information from bus 0 which
belongs to controller 0. Parser for 'pci regions' cmdline currently ignores
any additional arguments and so U-Boot always uses bus 0.

Regions are stored in controller (not on the bus) and therefore to retrieve
controller from the bus, it is needed to call pci_get_controller() which
returns root bus. Because bus 0 is root bus, current code worked fine for
controller 0.

Extend cmdline parser for 'pci regions' to allows specifying bus number,
extend pci_show_regions() code to accept also non-zero bus number and
print bus ranges for which is regions configuration assigned.

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