soc: qcom: mdt_loader: define stubs for COMPILE_TEST

Define stub functions for the exposed MDT functions in case
QCOM_MDT_LOADER is not configured.  This allows users of these
functions to link correctly for COMPILE_TEST builds without
QCOM_SCM enabled.

Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>