soc: qcom: rmtfs-mem: Validate that scm is available

The scm device must be present in order for the rmtfs driver to
configure memory permissions for the rmtfs memory region, so check that
it is probed before continuing.

Cc: stable@vger.kernel.org
Fixes: fa65f804 ("soc: qcom: rmtfs-mem: Add support for assigning memory to remote")
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>