irqchip/stm32: Add suspend support

This patch adds suspend feature.
-Use default irq_set_wake function to store wakeup request.
-Suspend function set wake_active into imr of each bank
 and save rising/falling trigger registers.
-Resume function restore the mask_cache interrupt into
 imr of each bank and restore rising/falling trigger registers.

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