hw/misc: In STM32L4x5 EXTI, correct configurable interrupts

hw/misc: In STM32L4x5 EXTI, correct configurable interrupts

The implementation of configurable interrupts (interrupts supporting
edge selection) was incorrectly expecting alternating input levels :
this commits adds a new status field `irq_levels` to actually detect
edges.

Signed-off-by: Inès Varhol <ines.varhol@telecom-paris.fr>
Message-id: 20240629110800.539969-2-ines.varhol@telecom-paris.fr
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>