soc: qcom: scm: Add APIs to allow register access via the secure world

Add an API to allow certain registers to be accessed
via secure world syscalls. Switch existing users to
this new API.

Change-Id: I1c2a67f728ddf486e93751e77853cc50e1b9db21
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>