irqchip/stm32-exti: add missing DT IRQ flag translation

The STM32F4/7 EXTI driver was missing the xlate callback, so IRQ trigger
flags specified in the device tree were being ignored. This was
preventing the RTC alarm interrupt from working, because it must be set
to trigger on the rising edge to function correctly.

Signed-off-by: Ben Wolsieffer <ben.wolsieffer@hefring.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231003162003.1649967-1-ben.wolsieffer@hefring.com