Add ChangeLog for rng removal

Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
diff --git a/ChangeLog.d/removal-of-rng.txt b/ChangeLog.d/removal-of-rng.txt
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+++ b/ChangeLog.d/removal-of-rng.txt
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+API changes
+   * All API functions now use the PSA random generator psa_get_random() 
+     internally. As a consequence, functions no longer take RNG parameters.
+     Please refer to the migration guide at :
+     tf-psa-crypto/docs/4.0-migration-guide.md.