firmware: scmi: mailbox transport: use plat data, not priv data

Change SCMI mailbox transport drivers to use platform data rather
than private data for channel reference since it only stores platform
data retrieved from the DT. Consequently the probe handler is replaced
with a of_to_plat handler.

Cc: Simon Glass <sjg@chromium.org>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>