pcie: Add API for scanning for available endpoints

This adds a generic API to be used for scanning for available PCI
endpoints. It takes a more detailed approach than the "brute force"
based scanning that's so far been used in Zephyr, buy inspecting the
host controller node and bridge nodes, and only scanning for busses and
devices that are actually expected to exist.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>