docs: mark STM32MP_USE_STM32IMAGE as deprecated

This macro was used for the legacy boot mode on SPM32MP platforms.
The recommended boot method is now FIP.
The code under this macro will be removed after tag v2.7.

Change-Id: Id3b7baea2d3e6ea8b36a4cd0b107cb92591a172b
Signed-off-by: Yann Gautier <yann.gautier@st.com>
diff --git a/docs/about/release-information.rst b/docs/about/release-information.rst
index b65571d..a50b033 100644
--- a/docs/about/release-information.rst
+++ b/docs/about/release-information.rst
@@ -61,7 +61,7 @@
 |                                | Date        | after   |                                                         |
 |                                |             | Release |                                                         |
 +================================+=============+=========+=========================================================+
-|                                |             |         |                                                         |
+| STM32MP_USE_STM32IMAGE macro   |   Dec '21   |   2.7   | FIP is the recommended boot method for STM32MP          |
 +--------------------------------+-------------+---------+---------------------------------------------------------+
 
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