ARM: dts: stm32: fix pinctrl node name warnings (MPU soc)

The recent addition pinctrl.yaml in commit c09acbc4 ("dt-bindings:
pinctrl: use pinctrl.yaml") resulted in some node name warnings.
Fix the node names to the preferred 'pinctrl'.

Signed-off-by: Fabien Dessenne <fabien.dessenne@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>