Enable SRE_EL2 on ARM64

Enable system register access for EL2. Alpine-V2 is
the first device requiring this to be enabled.
It is also in-sync with Linux initialization code,
and compatible with Alpine-V2 uboot requirements.

Obtained from: Semihalf
Submitted by: Michal Stanek <mst@semihalf.com>
Sponsored by: Annapurna Labs
Approved by: cognet (mentor)
Reviewed by: wma
Differential revision: https://reviews.freebsd.org/D5394