irqchip/stm32: Add suspend/resume support for hierarchy domain

This patch adds suspend/resume feature for exti hierarchy domain.
-suspend function sets wake_active into imr of each banks
-resume function restores the mask_cache interrupt into
 imr of each banks

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