dt-bindings: remoteproc: stm32_rproc: update for firmware synchronization

dt-bindings: remoteproc: stm32_rproc: update for firmware synchronization

Add new properties description used to attach to a pre-loaded
firmware according to the commit 9276536f455b3
("remoteproc: stm32: Parse syscon that will manage M4 synchronisation")
which updates the driver part.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Link: https://lore.kernel.org/r/20201014125441.2457-3-arnaud.pouliquen@st.com
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>