dt-bindings: interrupt-controller: stm32-exti: document st,stm32mp13-exti

Support of STM32MP13 SoC implies to create a new compatible in order to
manage EXTI/GIC mapping changes.

Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220202140005.860-2-alexandre.torgue@foss.st.com