remoteproc: stm32: add an ST stm32_rproc driver

This patch introduces a new remoteproc driver to control Cortex-M4
co-processor of the STM32 family.
It provides with the following features:
- start and stop
- dedicated co-processor memory regions registration
- coredump and recovery

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
[bjorn: Fixup of dev_dbg types and cast of int to pointer in mbox send]
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>