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>

arch/arm/boot/dts/stm32mp131.dtsi

@@ -75,6 +75,12 @@ clk_pll4_r: clk-pll4_r {
 			compatible = "fixed-clock";
 			clock-frequency = <99000000>;
 		};
+
+		clk_rtc_k: clk-rtc-k {
+			#clock-cells = <0>;
+			compatible = "fixed-clock";
+			clock-frequency = <32768>;
+		};
 	};
 
 	intc: interrupt-controller@a0021000 {
@@ -218,6 +224,15 @@ iwdg2: watchdog@5a002000 {
 			status = "disabled";
 		};
 
+		rtc: rtc@5c004000 {
+			compatible = "st,stm32mp1-rtc";
+			reg = <0x5c004000 0x400>;
+			interrupts-extended = <&exti 19 IRQ_TYPE_LEVEL_HIGH>;
+			clocks = <&clk_pclk4>, <&clk_rtc_k>;
+			clock-names = "pclk", "rtc_ck";
+			status = "disabled";
+		};
+
 		bsec: efuse@5c005000 {
 			compatible = "st,stm32mp15-bsec";
 			reg = <0x5c005000 0x400>;