irqchip/stm32-exti: Add domain translate function

Domain translate function is needed to recover irq
configuration parameters from DT node

Fixes: 927abfc4

 ("irqchip/stm32: Add stm32mp1 support with hierarchy domain")
Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>