hw/misc: In STM32L4x5 EXTI, handle direct interrupts

hw/misc: In STM32L4x5 EXTI, handle direct interrupts

The previous implementation for EXTI interrupts only handled
"configurable" interrupts, like those originating from STM32L4x5 SYSCFG
(the only device currently connected to the EXTI up until now).

In order to connect STM32L4x5 USART to the EXTI, this commit adds
handling for direct interrupts (interrupts without configurable edge).

Signed-off-by: Inès Varhol <ines.varhol@telecom-paris.fr>
Message-id: 20240707085927.122867-3-ines.varhol@telecom-paris.fr
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>