dm: pci: Allow a PCI bus to be found without an alias

dm: pci: Allow a PCI bus to be found without an alias

At present, until a PCI bus is probed, it cannot be found by its sequence
number unless it has an alias. This is the same with any device.

However with PCI this is more annoying than usual, since bus 0 is always the
same device.

Add a function that tries a little harder to locate PCI bus 0. This means
that PCI enumeration will happen automatically on the first access.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>