feat(stm32mp2-fdts): 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.

Change-Id: Icd1351e20b862675d257dede55df190a90acbd59
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>
diff --git a/fdts/stm32mp255.dtsi b/fdts/stm32mp255.dtsi
new file mode 100644
index 0000000..14164ce
--- /dev/null
+++ b/fdts/stm32mp255.dtsi
@@ -0,0 +1,9 @@
+// SPDX-License-Identifier: (GPL-2.0-or-later OR BSD-3-Clause)
+/*
+ * Copyright (C) 2023, STMicroelectronics - All Rights Reserved
+ * Author: Alexandre Torgue <alexandre.torgue@foss.st.com> for STMicroelectronics.
+ */
+#include "stm32mp253.dtsi"
+
+/ {
+};