doc: clarify scmi device tree for stm32mp15 boards

Clarify the usage of SCMI specific device tree to use with
stm32mp15_defconfig and with OP-TEE.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/doc/board/st/stm32mp1.rst b/doc/board/st/stm32mp1.rst
index 63b4477..8cf7129 100644
--- a/doc/board/st/stm32mp1.rst
+++ b/doc/board/st/stm32mp1.rst
@@ -180,35 +180,54 @@
 
 STM32MP15x device Tree Selection
 ````````````````````````````````
-The supported device trees for STM32MP15x (stm32mp15_trusted_defconfig and stm32mp15_basic_defconfig) are:
+The supported device trees for STM32MP15x (**stm32mp15_defconfig** for TF-A_
+with FIP support) are:
 
 + ev1: eval board with pmic stpmic1 (ev1 = mother board + daughter ed1)
 
+   + **stm32mp157c-ev1-scmi**
    + stm32mp157c-ev1
 
 + ed1: daughter board with pmic stpmic1
 
+   + **stm32mp157c-ed1-scmi**
    + stm32mp157c-ed1
 
 + dk1: Discovery board
 
+   + **stm32mp157a-dk1-scmi**
    + stm32mp157a-dk1
 
 + dk2: Discovery board = dk1 with a BT/WiFI combo and a DSI panel
 
+   + **stm32mp157c-dk2-scmi**
    + stm32mp157c-dk2
 
+The scmi variant of each device tree is only supported with OP-TEE as secure
+monitor and it is the configuration **recommended** by STMicroelectronics for
+product, with secured system resources (RCC_TZCR.TZEN=1).
+
+The supported device trees for STM32MP15x (stm32mp15_trusted_defconfig
+TF-A without FIP support and stm32mp15_basic_defconfig with SPL) are:
+
++ the same STMicroelectronics boards with the no scmi device tree files:
+
+   + stm32mp157c-ev1
+   + stm32mp157c-ed1
+   + stm32mp157a-dk1
+   + stm32mp157c-dk2
+
 + avenger96: Avenger96 board from Arrow Electronics based on DH Elec. DHCOR SoM
 
    + stm32mp15xx-dhcor-avenger96
 
 STM32MP13x device Tree Selection
 ````````````````````````````````
-The supported device trees for STM32MP13x (stm32mp13_defconfig) are:
+The supported device trees for STM32MP13x (**stm32mp13_defconfig**) are:
 
 + dk: Discovery board
 
-   + stm32mp135f-dk
+   + **stm32mp135f-dk**
 
 
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