RFC: Device overlay manager (PCI/USB + DT)

This probably misplaced (in drivers/misc) patch allows use
of device tree overlays on the two kinds of probeable busses
that count nowadays, PCI & USB.

It does so by dynamically creating device nodes for the busses &
devices that are probed and according to user-configuration applying
an overlay when they appear.

It is still a WIP but it's coming along nicely.

Issues: Only PCI works for now, the generated bindings are not
correct according to the openfirmware spec.

Signed-off-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>