dm: pci: Allow PCI bus numbering aliases

Commit 9cc36a2b 'dm: core: Add a flag to control sequence numbering' changed
the default uclass behaviour to not support bus numbering. This is incorrect
for PCI and that commit should have enabled the flag for PCI.

Enable it so that PCI buses can be found and the 'pci' command works again.
Also add a test for this.

Signed-off-by: Simon Glass <sjg@chromium.org>