psa: Rename functions to get a key slot

Rename functions to get a key slot:
. to make their naming more consistent
. to emphasize that those functions set a lock on the
  key slot they return to protect it from being wiped
  out and re-used while some part of the library
  is accessing it.

Signed-off-by: Ronald Cron <ronald.cron@arm.com>
diff --git a/include/psa/crypto_extra.h b/include/psa/crypto_extra.h
index 7986eb2..0c90cb2 100644
--- a/include/psa/crypto_extra.h
+++ b/include/psa/crypto_extra.h
@@ -231,8 +231,8 @@
     size_t cache_slots;
     /** Number of slots that are not used for anything. */
     size_t empty_slots;
-    /** Number of slots that are not accessed. */
-    size_t unaccessed_slots;
+    /** Number of slots that are not locked. */
+    size_t unlocked_slots;
     /** Largest key id value among open keys in internal persistent storage. */
     psa_key_id_t max_open_internal_key_id;
     /** Largest key id value among open keys in secure elements. */