remoteproc: stm32: clear MCU PDDS at firmware start

When remoteproc stops a firmware, it sets the MCU PDDS flag, allowing
the platform to reach the Standby power mode if needed.
Symmetrically, clear this flag before running a firmware.
This fixes the issue where the system goes to Standby mode when the
MCU crashes while MPU is in CSTOP mode.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>