ARM: stm32: Increment WDT by default on DHSOM

Enable watchdog timer on the DHSOM by default, both in U-Boot proper and
in SPL. This can be used in combination with boot counter by either SPL
or U-Boot proper to boot either copy of system software, e.g. in case of
full A/B update strategy.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>