pinctrl: stm32: Add level interrupt support to gpio irq chip

GPIO hardware block is directly linked to EXTI block but EXTI handles
external interrupts only on edge. To be able to handle GPIO interrupt on
level a "hack" is done in gpio irq chip: parent interrupt (exti irq chip)
is retriggered following interrupt type and gpio line value.

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Tested-by: Marek Vasut <marex@denx.de>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200219143229.18084-3-alexandre.torgue@st.com