qcom: add initial SCM legacy API

This is a very simple implementation of Qualcomm's SCM API.

It is just the structure/field definitions and the atomic SCM
call which doesn't use the structs yet - it uses the field
definitions inside registers.

I've tested that setting the cold boot address via the atomic
API is fine - Linux does the same thing. But not all SCM calls
can be done via the legacy API.

This is a reimplementation based on the Linux qualcomm SCM legacy
code and definitions.

Tested:

• Qualcomm IPQ4018 AP, as part of other changes for doing SMP bring-up

Reviewed by: andrew, manu, imp
Differential Revision: https://reviews.freebsd.org/D32723