Add the arch field to the arm64 MIDR macros

For completeness add accessors for the MIDR field. As the field is
always 0xf on arm64 it is unneeded in the current MICR handling, but
will be used in the vmm module for bhyve.

Obtained from: https://github.com/FreeBSD-UPB/freebsd-src (earlier version)
Sponsored by: The FreeBSD Foundation