arm64: Check if PSCI before calling SMCCC

As SMCCC depends on PSCI check if the latter is present before calling
the former. This fixes an issue where we may call into SMCCC when there
is no PSCI in the system causing a smccc_version assert to fail.

Reported by: stevek
Reviewed by: emaste, imp, stevek
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42404