[kernel][arch] Give 'fieldless' CPUID types an uber-field

In the case of a register type that has no subfield, cosmetically
define a field ranging over all bits with a contextually sensible name.
For example, we give the 'max leaf' types as single "leaf" field.

Bug: 61093
Change-Id: Ibb601074b2add529aeea6de5919f69db9816f812
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/464634
Commit-Queue: Joshua Seaton <joshuaseaton@google.com>
Reviewed-by: Roland McGrath <mcgrathr@google.com>