ARM: dts: stm32: add cpu clock-frequency property on stm32mp15x

All of the STM32MP151[1], STM32MP153[2] and STM32MP157[3] have their
Cortex-A7 cores running at 650 MHz.

Add the clock-frequency property to CPU nodes to avoid warnings about
them missing.

[1]: https://www.st.com/en/microcontrollers-microprocessors/stm32mp151.html
[2]: https://www.st.com/en/microcontrollers-microprocessors/stm32mp153.html
[3]: https://www.st.com/en/microcontrollers-microprocessors/stm32mp157.html

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32mp151.dtsi

@@ -17,6 +17,7 @@
 
 		cpu0: cpu@0 {
 			compatible = "arm,cortex-a7";
+			clock-frequency = <650000000>;
 			device_type = "cpu";
 			reg = <0>;
 		};

arch/arm/boot/dts/stm32mp153.dtsi

@@ -10,6 +10,7 @@
 	cpus {
 		cpu1: cpu@1 {
 			compatible = "arm,cortex-a7";
+			clock-frequency = <650000000>;
 			device_type = "cpu";
 			reg = <1>;
 		};