changelog: document RSA parse/write improvements

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/ChangeLog.d/8647.txt b/ChangeLog.d/8647.txt
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+Default behavior changes
+   * Importing of RSA keys in PEM format in PSA is officially unsupported
+     (this was previously undocumented).
+
+Features
+   * It is possible to enable RSA support in PSA (MBEDTLS_PSA_CRYPTO_C +
+     RSA_C) without enabling PK module (MBEDTLS_[PK|PK_WRITE|PK_PARSE]_C).