irqchip/stm32: Add irq retrigger support

This commit introduces retrigger support for stm32_ext_h chip.
It consists to rise the GIC interrupt mapped to an EXTI line.

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20200219143229.18084-2-alexandre.torgue@st.com