Allow psci.h to be used by userspace

Wrap parts of psci.h that aren't usable by userspace in _KERNEL checks.
This allows it to be used to implement PSCI and SMCCC by bhyve in
userspace.

Sponsored by: Arm Ltd
Sponsored by: Innovate UK
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 473ab212dc1f72c1765f112568237d229d4e0441)