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/stm32mpus.rst b/docs/plat/st/stm32mpus.rst
index e09d25d..931dd57 100644
--- a/docs/plat/st/stm32mpus.rst
+++ b/docs/plat/st/stm32mpus.rst
@@ -4,7 +4,7 @@
 STM32 MPUs are microprocessors designed by STMicroelectronics
 based on Arm Cortex-A. This page presents the common configuration of STM32
 MPUs, more details and dedicated configuration can be found in each STM32 MPU
-page (:ref:`STM32MP1`)
+page (:ref:`STM32MP1` or :ref:`STM32MP2`)
 
 Design
 ------