Add mcontrol6. (#538)

* Add mcontrol6.
Mcontrol6 is like mcontrol but without a maskmax field and with a sizehi field on RV32.
A recipe is provided for determining the max mask when mcontrol6.match=1 (NAPOT).

* Apply suggestions from code review

Co-authored-by: Ernie Edgar <43148441+ernie-sifive@users.noreply.github.com>

* Added caveat about data value matches larger than XLEN.

* Review feedback:
- don't duplicate timing table. mcontrol6 is authoritative.
- use register/field macros
- unify size field in mcontrol6

* Revert PDF

Co-authored-by: Ernie Edgar <43148441+ernie-sifive@users.noreply.github.com>