dts/bindings: stm32: Set pinctrl-[0/names] properties as required

Now that STM32 drivers are using pinctrl API, set pintrl-0 and
pintrl-names properties as required in order to report malformed
nodes description soon at build stage and avoid cryptic
DT api build error messages.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>