arm64: Update how we handle SpecSEI

The ID_AA64MMFR1_EL0.SpecSEI field needs to use MRS_HIGHER to get the
largest value from all CPUs. This is because it indicates when an
exception might happen when it's non-zero and can't happen when zero.
As indicating something that might happen even when it can't is safer
use MRS_HIGHER to handle this field.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47126