Simplify implementation of MD<->PSA translation

Also, add tests and comments due from previous commits.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/library/md_psa.h b/library/md_psa.h
index 3231a60..2f6c701 100644
--- a/library/md_psa.h
+++ b/library/md_psa.h
@@ -33,6 +33,9 @@
  *
  * \param md_type   The type of digest to search for.
  *
+ * \warning         This function does not check if the algorithm is
+ *                  supported, it always returns the corresponding identifier.
+ *
  * \return          The PSA algorithm identifier associated with \p md_type,
  *                  regardless of whether it is supported or not.
  */
@@ -44,6 +47,9 @@
  *
  * \param psa_alg   The PSA algorithm identifier to search for.
  *
+ * \warning         This function does not check if the algorithm is
+ *                  supported, it always returns the corresponding identifier.
+ *
  * \return          The MD type associated with \p psa_alg,
  *                  regardless of whether it is supported or not.
  */