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