watchdog: don't autostart watchdog on STM32MP architecture

watchdog: don't autostart watchdog on STM32MP architecture

On STM32MP reference boards, the watchdog is started by a previous
boot stage (e.g. bootrom or secure OS), so the config flag
WATCHDOG_AUTOSTART is not required.
It's preferable to rely on the DT properties "u-boot,autostart" or
"u-boot,noautostart", if needed.

For backward compatibility on defconfigs that are based on SPL,
thus cannot rely on a previous boot stage for starting the
watchdog, enable WATCHDOG_AUTOSTART in their respective defconfig.
The change in stm32mp15_dhsom.config is propagated to:
- configs/stm32mp15_dhcom_basic.config
- configs/stm32mp15_dhcor_basic.config
and then to:
- stm32mp15_dhcom_basic_defconfig
- stm32mp15_dhcom_drc02_basic_defconfig
- stm32mp15_dhcom_pdk2_basic_defconfig
- stm32mp15_dhcom_picoitx_basic_defconfig
- stm32mp15_dhcor_avenger96_basic_defconfig
- stm32mp15_dhcor_basic_defconfig
- stm32mp15_dhcor_drc_compact_basic_defconfig
- stm32mp15_dhcor_testbench_basic_defconfig

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>