irqchip/stm32: Add stm32mp1 support with hierarchy domain

Exti controller has been differently integrated on stm32mp1 SoC.
A parent irq has only one external interrupt. A hierachy domain could
be used. Handlers are call by parent, each parent interrupt could be
masked and unmasked according to the needs.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>