docs: update STM32MP1 with versions details

After introducing the new STM32MP1 SoC versions in patch [1], the
document describing STM32MP1 platform is updated with the information
given in the patch commit message.

 [1]: stm32mp1: add support for new SoC profiles

Change-Id: I6d7ce1a3c29678ddac78a6685f5d5daf28c3c3a1
Signed-off-by: Yann Gautier <yann.gautier@st.com>

docs/plat/stm32mp1.rst

@@ -8,6 +8,23 @@ The STM32MP1 chip also embeds a Cortex-M4.
 More information can be found on `STM32MP1 Series`_ page.
 
 
+STM32MP1 Versions
+-----------------
+The STM32MP1 series is available in 3 different lines which are pin-to-pin compatible:
+
+- STM32MP157: Dual Cortex-A7 cores, Cortex-M4 core @ 209 MHz, 3D GPU, DSI display interface and CAN FD
+- STM32MP153: Dual Cortex-A7 cores, Cortex-M4 core @ 209 MHz and CAN FD
+- STM32MP151: Single Cortex-A7 core, Cortex-M4 core @ 209 MHz
+
+Each line comes with a security option (cryptography & secure boot) and a Cortex-A frequency option:
+
+- A      Basic + Cortex-A7 @ 650 MHz
+- C      Secure Boot + HW Crypto + Cortex-A7 @ 650 MHz
+- D      Basic + Cortex-A7 @ 800 MHz
+- F      Secure Boot + HW Crypto + Cortex-A7 @ 800 MHz
+
+The `STM32MP1 part number codification`_ page gives more information about part numbers.
+
 Design
 ------
 The STM32MP1 resets in the ROM code of the Cortex-A7.
@@ -129,3 +146,4 @@ OP-TEE artifacts go into separate partitions as follows:
 
 
 .. _STM32MP1 Series: https://www.st.com/en/microcontrollers-microprocessors/stm32mp1-series.html
+.. _STM32MP1 part number codification: https://wiki.st.com/stm32mpu/wiki/STM32MP15_microprocessor#Part_number_codification