docs(stm32mp2): introduce new STM32MP21 family

STM32MP21 is a derivative from STM32MP25. It comes in 3 different lines:
- STM32MP215: Single Cortex-A35 + Cortex-M33 - 2x Ethernet - 2x CAN FD
              CSI - LTDC
- STM32MP213: Single Cortex-A35 + Cortex-M33 - 2x Ethernet - 2x CAN FD
- STM32MP211: Single Cortex-A35 + Cortex-M33 - 1x Ethernet

Change-Id: Ie3db430bedd86c3b444bec647792be24b20a0cba
Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>
diff --git a/docs/plat/st/stm32mp2.rst b/docs/plat/st/stm32mp2.rst
index 87bb6a5..d63fc50 100644
--- a/docs/plat/st/stm32mp2.rst
+++ b/docs/plat/st/stm32mp2.rst
@@ -12,6 +12,27 @@
 STM32MP2 Versions
 -----------------
 
+Here are the variants for STM32MP2:
+- STM32MP21
+- STM32MP25
+
+STM32MP21 Versions
+~~~~~~~~~~~~~~~~~~
+The STM32MP21 series is available in 3 different lines which are pin-to-pin compatible:
+
+- STM32MP215: Single Cortex-A35 + Cortex-M33 - 2x Ethernet - 2x CAN FD - CSI - LTDC
+- STM32MP213: Single Cortex-A35 + Cortex-M33 - 2x Ethernet - 2x CAN FD
+- STM32MP211: Single Cortex-A35 + Cortex-M33 - 1x Ethernet
+
+Each line comes with a security option (cryptography & secure boot) and a Cortex-A frequency option:
+
+- A      Basic + Cortex-A35 @ 1.2GHz
+- C      Secure Boot + HW Crypto + Cortex-A35 @ 1.2GHz
+- D      Basic + Cortex-A35 @ 1.5GHz
+- F      Secure Boot + HW Crypto + Cortex-A35 @ 1.5GHz
+
+STM32MP25 Versions
+~~~~~~~~~~~~~~~~~~
 The STM32MP25 series is available in 4 different lines which are pin-to-pin compatible:
 
 - STM32MP257: Dual Cortex-A35 cores, Cortex-M33 core - 3x Ethernet (2+1 switch) - 3x CAN FD – H264 - 3D GPU – AI / NN - LVDS
@@ -70,6 +91,8 @@
 
 - | ``STM32MP_DDR_FIP_IO_STORAGE``: to store DDR firmware in FIP.
   | Default: 1
+- | ``STM32MP21``: to select STM32MP21 variant configuration.
+  | Default: 0
 - | ``STM32MP25``: to select STM32MP25 variant configuration.
   | Default: 1
 
@@ -154,4 +177,4 @@
 .. _STM32MP2 part number codification: https://wiki.st.com/stm32mpu/wiki/STM32MP25_microprocessor#Part_number_codification
 .. _STMicroelectronics DDR PHY github: https://github.com/STMicroelectronics/stm32-ddr-phy-binary
 
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+*Copyright (c) 2023-2025, STMicroelectronics - All Rights Reserved*