msm: Secure Channel Manager (SCM) support

SCM is the protocol used to communicate between the secure and
non-secure code executing on the applications processor. The
non-secure side uses a physically contiguous buffer to pass
information to the secure side; where the buffer conforms to a
format that is agreed upon by both sides. The use of a buffer
allows multiple pending requests to be in flight on the secure
side. It also benefits use cases where the command or response
buffer contains large chunks of data.

Reviewed-by: Saravana Kannan <skannan@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: David Brown <davidb@codeaurora.org>