ARM: dts: stm32: add EXTI interrupt-parent to pinctrl node on stm32mp131

Add interrupt-parent property in pinctrl node to use GPIO as IRQ.

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