irqchip/stm32: Add multi-bank management

-Prepare to manage multi-bank of external interrupts
(N banks of 32 inputs).
-Prepare to manage registers offsets by compatible
(registers offsets could be different follow per stm32 platform).

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