MFC 303886: Add additional constants.

MFC 303886: Add additional constants.

- Add constants for the fields in the root-entry table address register,
  namely the root type type (RTT) and root table address (RTA) mask.
- Add macros for the bitmask of the domain ID field in the second word
  of context table entries as well as a helper macro (DMAR_CTX2_GET_DID)
  to extract the domain ID from a context table entry.

Sponsored by:	Chelsio Communications

git-svn-id: svn://svn.freebsd.org/base/stable/10@306466 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f