hw/misc: In STM32L4x5 EXTI, consolidate 2 constants

hw/misc: In STM32L4x5 EXTI, consolidate 2 constants

Up until now, the EXTI implementation had 16 inbound GPIOs connected to
the 16 outbound GPIOs of STM32L4x5 SYSCFG.
The EXTI actually handles 40 lines (namely 5 from STM32L4x5 USART
devices which are already implemented in QEMU).
In order to connect USART devices to EXTI, this commit consolidates
constants `EXTI_NUM_INTERRUPT_OUT_LINES` (40) and
`EXTI_NUM_GPIO_EVENT_IN_LINES` (16) into `EXTI_NUM_LINES` (40).

Signed-off-by: Inès Varhol <ines.varhol@telecom-paris.fr>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20240707085927.122867-2-ines.varhol@telecom-paris.fr
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>