arm64: Panic if the ID register isn't known

This will allow for callers to be simplified & not need to check the
return status.

Keep the return type for now so this can be MFCd without breaking the
KBI.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D55104

(cherry picked from commit 3b0b6aa2cd05b516ecf4b72f9a3c2aadddf3c94e)