uefi: Add device path generation for discovered devices in a PciTree

[seijikun]

Initial implementation for device path generation of discovered pci devices in a PciTree instance.
Unfortunately, it's a bit clunky to use, since you have to query the DevicePath corresponding to your PciRootBridgeIo instance yourself and pass that in:

let mut pci_proto = get_open_protocol::<PciRootBridgeIo>(pci_handle);
let root_device_path = get_open_protocol::<DevicePath>(pci_handle);

let pci_tree = pci_proto.enumerate().unwrap();
for addr in pci_tree {
    // v Would be nicer, if I didn't have to pass root DevicePath in manually here v
    let device_path = pci_tree.device_path(&root_device_path, addr).unwrap();
}

I tried getting the DevicePath of the PciRootBridgeIo instance within PciRootBridgeIo::enumerate() and pass that on to the PciTree, but I couldn't find a way to get the DevicePath instance - other than ugly workarounds.

Best consumed Commit-By-Commit. Contains #1830..

Steps to Undraft

• merge and rebase onto uefi: Refactor PciRootBridgeIo::enumerate() with tree-topology information #1830

Checklist

• Sensible git history (for example, squash "typo" or "fix" commits). See the Rewriting History guide for help.
• Update the changelog (if necessary)