ARM: dts: stm32: add RTC support on STM32H743

This patch adds support for RTC on STM32H743 SoC.
It also adds dt-bindings/interrupt-controller/irq.h include and uses it to
configure RTC alarm interrupt.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>