Add support for the ARM SMC Calling Convention (SMCCC). This is a method
to call into the firmware in a similar way to the existing PSCI, and used
PSCI to detect when SMCCC is enabled.

There is a function ID space we can use. Currently we only support 3
functions in the ARM Architecture Calls region, however it is expected we
will expend these in the future.

Sponsored by: DARPA, AFRL