docs: deprecate io_dummy driver

This drivers was only used by one upstream platform: STM32MP1 but only
when enabling the flag STM32MP_USE_STM32IMAGE. This flag and the
corresponding code is now removed from TF-A. The driver can then be
set to deprecated. It will be removed after v2.9 tag.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Ib8242a7291c7011d7f96a6a83609ec1996dce520
diff --git a/docs/about/release-information.rst b/docs/about/release-information.rst
index ddfc081..f99b7ff 100644
--- a/docs/about/release-information.rst
+++ b/docs/about/release-information.rst
@@ -69,6 +69,8 @@
 +================================+=============+=========+=========================================================+
 | plat_convert_pk() function     |   Nov'22    |   2.9   | Platform conversion to manage specific PK hash          |
 +--------------------------------+-------------+---------+---------------------------------------------------------+
+| io_dummy driver                |   Nov'22    |   2.9   | No more used by any upstream platform                   |
++--------------------------------+-------------+---------+---------------------------------------------------------+
 
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