irqchip/stm32-exti: Add STM32MP15xx IWDG2 EXTI to GIC map

The EXTI interrupt 46 is mapped to GIC interrupt 151. Add the
missing mapping, which is used for IWDG2 pretimeout interrupt
and wake up source.

Reviewed-by: Antonio Borneo <antonio.borneo@foss.st.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230517194349.105745-1-marex@denx.de