remoteproc: stm32: Parse syscon that will manage M4 synchronisation

Get from the DT the syncon to probe the state of the remote processor
and the location of the resource table.

Mainly based on the work published by Arnaud Pouliquen [1].

[1]. https://patchwork.kernel.org/project/linux-remoteproc/list/?series=239877



Acked-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Reviewed-by: Loic Pallardy <loic.pallardy@st.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20200714200445.1427257-6-mathieu.poirier@linaro.org


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>