STUSB1600 IRQ (Alert pin) is active low (open drain). Interrupts may get
lost currently, so fix the IRQ type.

Fixes: 83686162 ("ARM: dts: stm32: add STUSB1600 Type-C using I2C4 on stm32mp15xx-dkx")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>