msm-pm: Add support for the new scm_call2 API

The scm library has added support for a new secure world
interface that is more aligned to the ARMv8 SMC calling
convention. Use the new API while maintaining backward
compatibility.

Change-Id: I1e80464e981b86428b80f7676f9a8aba6b6c869f
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>