Move attribute fields to a substructure

Move the "core attributes" to a substructure of psa_key_attribute_t.
The motivation is to be able to use the new structure
psa_core_key_attributes_t internally.
diff --git a/include/psa/crypto_extra.h b/include/psa/crypto_extra.h
index b2d4633..6dfaa13 100644
--- a/include/psa/crypto_extra.h
+++ b/include/psa/crypto_extra.h
@@ -89,7 +89,7 @@
     psa_key_attributes_t *attributes,
     psa_algorithm_t alg2)
 {
-    attributes->policy.alg2 = alg2;
+    attributes->core.policy.alg2 = alg2;
 }
 
 /** Retrieve the enrollment algorithm policy from key attributes.
@@ -101,7 +101,7 @@
 static inline psa_algorithm_t psa_get_key_enrollment_algorithm(
     const psa_key_attributes_t *attributes)
 {
-    return( attributes->policy.alg2 );
+    return( attributes->core.policy.alg2 );
 }
 
 /**@}*/