samples: benchmarks: coremark: indicate the broken nRF54H20 DK target

Added a note to the CoreMark sample documentation to warn the
user that the nRF54H20 DK no longers works with this project.

Signed-off-by: Kamil Piszczek <[email protected]>

Author: kapi-no

samples/benchmarks/coremark/README.rst

@@ -17,6 +17,11 @@ The sample supports the following development kits:
 
 .. table-from-sample-yaml::
 
+.. note::
+   This sample does not build or run for the ``nrf54h20dk/nrf54h20/cpuapp`` board target due to the IronSide SE migration.
+   See the ``NCSDK-34698`` in the :ref:`known_issues` page for more information.
+   The codebase and documentation will be updated in the future releases to address this issue.
+
 Overview
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