arm64: Add constants for decoding ISS fields for WF* exceptions

WFI and WFIT trap to EL2 when executed in a vmm guest. (Currently
WFE/WFET are not configured to trap.) We only handle WFI at the moment,
so these constants are useful when handling the exception.

Reviewed by: andrew
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D41199