configs: stm32mp1: activate WATCHDOG

As kernel v5.6 have a solution since so we will be able to enable
the watchdog at boot time. It is reloaded by the watchdog
framework (if CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED is set) and
until the userspace watchdog daemon takes over control.

Need presence of kernel patch 85fdc63fe256 ("drivers: watchdog:
stm32_iwdg: set WDOG_HW_RUNNING at probe") integrated in v5.6-rc1.

This patch revert the previous commit ca351e70 ("stm32mp1:
deactivate WATCHDOG in defconfig").

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>