ARM: dts: stm32: use exti 19 as main interrupt to support RTC wakeup on stm32mp157

Link between GIC and exti line is now done inside EXTI driver. So in order
to be wake up source exti irqchip has to be used.

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>