feat(docs): introduce STM32MP2 doc

STM32MP2x is a new family of microprocessors designed by
STMicroelectronics and based on Arm Cortex-A35.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I741ed0a701a614817a4d0b65d3d6f4e6a79da6a9
diff --git a/docs/plat/st/index.rst b/docs/plat/st/index.rst
index 6e18bd7..95ec3d2 100644
--- a/docs/plat/st/index.rst
+++ b/docs/plat/st/index.rst
@@ -7,6 +7,7 @@
 
    stm32mpus
    stm32mp1
+   stm32mp2
 
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