ARM: dts: stm32: add RTC node on stm32mp131

Add RTC node with compatible, clock, and interrupt properties
on stm32mp131.

Add clk_rtc_k fixed clock for RTC.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@foss.st.com>
Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>