dm: pci: Add a way to iterate through all PCI devices

These functions allow iteration through all PCI devices including bridges.
The children of each PCI bus are returned in turn. This can be useful for
configuring, checking or enumerating all the devices.

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