add ChangeLog entry

Signed-off-by: Yanray Wang <yanray.wang@arm.com>
diff --git a/ChangeLog.d/add-aes-128bit-only.txt b/ChangeLog.d/add-aes-128bit-only.txt
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+++ b/ChangeLog.d/add-aes-128bit-only.txt
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+Features
+   * Add support for AES to only enable 128-bit key in order to save code size.
+     A new configuration option, MBEDTLS_AES_ONLY_128_BIT_KEY_LENGTH, can be
+     used to enable this feature.
+     Fixes #7376.