dt-bindings/interrupt-controllers: Add compatible string for stm32h7

This patch updates stm32-exti documentation with stm32h7-exti
compatible string.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>