Add generic arm/arm64 secure-monitor SMCCC interface and switch
PSCI code to use it.

This interface will also be used by Intel Stratix 10 platform.

This was not tested on arm due to lack of PSCI-enabled arm hardware
lying around.

Reviewed by: andrew
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D21439