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

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>