Provide macros to extract the sub-fields of the CAP_LO and CAP_HI registers.

These macros make places where we extract these easier to read. The shift and
mask stuff is also a bit tedious and error prone. Start with the CAP_LO and
CAP_HI registers since their scope is somewhat constrained. This is style
chagne only, no functional changes.

Reviewed by: chuck
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20979