Only PCI was treated as a "less specific" bus that should not be
assigned if USB was already assigned, so non-PCI USB controllers
(under platform/DT or ACPI) would have all their devices *not*
treated as USB at all, resulting in external peripherals being
treated as internal ones on DT platforms.
Clarify these assignments into specificity/nesting groups instead
of having some arbitrary "X must be before Y" comments.
Fixes a4381cae8b.
