Add Changelog entry

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/ChangeLog.d/add_psa_m_aead.txt b/ChangeLog.d/add_psa_m_aead.txt
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+++ b/ChangeLog.d/add_psa_m_aead.txt
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+Features
+   * Implemented the multipart AEAD API within the PSA Crypto API, along with
+     tests in the PSA Crypto test suite, and transparent driver wrappers.