remoteproc: qcom: mdt_loader: Refactor MDT loader

Pushing the SCM calls into the MDT loader reduces duplication in the
callers and allows for non-remoteproc clients to use the helper for
parsing and loading MDT files.

Cc: Andy Gross <andy.gross@linaro.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>