remoteproc: stm32: fix probe error case

remoteproc: stm32: fix probe error case

If the rproc driver is probed before the mailbox driver and if the rproc
Device Tree node has some mailbox properties, the rproc driver probe
shall be deferred instead of being probed without mailbox support.

Tested-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Link: https://lore.kernel.org/r/1573812188-19842-1-git-send-email-fabien.dessenne@st.com
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>