soc: qcom: smem: introduce qcom_smem_virt_to_phys()

Create function qcom_smem_virt_to_phys(), which returns the physical
address corresponding to a given SMEM item's virtual address.  This
feature is required for a driver that will soon be out for review.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>