arm64: dts: st: Add exti1 and exti2 nodes on stm32mp251

Update the device-tree stm32mp251.dtsi by adding the nodes for exti1 and
exti2 interrupt controllers.

Signed-off-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240415134926.1254428-11-antonio.borneo@foss.st.com