docs(stm32mp2): introduce new STM32MP23 family

STM32MP23 is a derivative from STM32MP25. It comes in 3 different lines:
- STM32MP235: Dual Cortex-A35 + Cortex-M33 - 2x Ethernet - 2x CAN FD
              H264 - 3D GPU - AI / NN - LVDS / DSI
- STM32MP233: Dual Cortex-A35 + Cortex-M33 - 2x Ethernet - 2x CAN FD
- STM32MP231: Single Cortex-A35 + Cortex-M33 - 1x Ethernet

Change-Id: Iaf3dd7e0c1eda055063361af3c98855b1272d4c6
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 d63fc50..e64b989 100644
--- a/docs/plat/st/stm32mp2.rst
+++ b/docs/plat/st/stm32mp2.rst
@@ -14,6 +14,7 @@
 
 Here are the variants for STM32MP2:
 - STM32MP21
+- STM32MP23
 - STM32MP25
 
 STM32MP21 Versions
@@ -31,6 +32,21 @@
 - D      Basic + Cortex-A35 @ 1.5GHz
 - F      Secure Boot + HW Crypto + Cortex-A35 @ 1.5GHz
 
+STM32MP23 Versions
+~~~~~~~~~~~~~~~~~~
+The STM32MP23 series is available in 3 different lines which are pin-to-pin compatible:
+
+- STM32MP235: Dual Cortex-A35 + Cortex-M33 - 2x Ethernet - 2x CAN FD - H264 - 3D GPU - AI / NN - LVDS / DSI
+- STM32MP233: Dual Cortex-A35 + Cortex-M33 - 2x Ethernet - 2x CAN FD
+- STM32MP231: 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:
@@ -93,6 +109,8 @@
   | Default: 1
 - | ``STM32MP21``: to select STM32MP21 variant configuration.
   | Default: 0
+- | ``STM32MP23``: to select STM32MP23 variant configuration.
+  | Default: 0
 - | ``STM32MP25``: to select STM32MP25 variant configuration.
   | Default: 1