New usage flag PSA_KEY_USAGE_COPY

Document the new flag and allow its use.
diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 6a4f180..b0acc30 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1344,6 +1344,7 @@
     const psa_key_policy_t *policy )
 {
     if( ( policy->usage & ~( PSA_KEY_USAGE_EXPORT |
+                             PSA_KEY_USAGE_COPY |
                              PSA_KEY_USAGE_ENCRYPT |
                              PSA_KEY_USAGE_DECRYPT |
                              PSA_KEY_USAGE_SIGN |