bus: Document special ranges of IVARs

Some IVAR indices are special in that they have global meaning across
multiple buses where as other IVARs are always private to the local bus.
Try to document t

bus: Document special ranges of IVARs

Some IVAR indices are special in that they have global meaning across
multiple buses where as other IVARs are always private to the local bus.
Try to document this a bit and add constants for the various ranges to
avoid future conflicts.

This is a no-op, but IVAR indices are now generally defined as enums
as that makes it easier to define them in terms of ranges.

Reviewed by: imp, royger, andrew
Differential Revision: https://reviews.freebsd.org/D54159

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