firmware: qcom: scm: add a dummy qcom_scm_assign_mem()

Add a dummy qcom_scm_assign_mem() to enable building drivers when
CONFIG_COMPILE_TEST=y && CONFIG_QCOM_SCM=n.

All other qcom_scm_* functions already have a dummy version.

Signed-off-by: Niklas Cassel <niklas.cassel@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>
