arm64: dts: st: introduce stm32mp25 SoCs family

STM32MP25 family is composed of 4 SoCs defined as following:

-STM32MP251: common part composed of 1*Cortex-A35, common peripherals like
SDMMC, UART, SPI, I2C, PCIe, USB3, parallel and DSI display, 1*ETH ...

-STM32MP253: STM32MP251 + 1*Cortex-A35 (dual CPU), a second ETH, CAN-FD and
LVDS display.

-STM32MP255: STM32MP253 + GPU/AI and video encode/decode.
-STM32MP257: STM32MP255 + ETH TSN switch (2+1 ports).

A second diversity layer exists for security features/ A35 frequency:
-STM32MP25xY, "Y" gives information:
-Y = A means A35@1.2GHz + no cryp IP and no secure boot.
-Y = C means A35@1.2GHz + cryp IP and secure boot.
-Y = D means A35@1.5GHz + no cryp IP and no secure boot.
-Y = F means A35@1.5GHz + cryp IP and secure boot.

Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>