msm: scm: Add API to query for service/command availability.

Some drivers may need to query the secure environment about the
availability of a particular service/command. Add support for
this.

Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
[sboyd@codeaurora.org: Add some commit text]

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>