Align names to use "opaque" only everywhere

It's better for names in the API to describe the "what" (opaque keys) rather
than the "how" (using PSA), at least since we don't intend to have multiple
function doing the same "what" in different ways in the foreseeable future.
diff --git a/library/pk.c b/library/pk.c
index f65b2ee..c34ab7e 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -143,9 +143,9 @@
 /*
  * Initialise a PSA-wrapping context
  */
-int mbedtls_pk_setup_psa( mbedtls_pk_context *ctx, const psa_key_slot_t key )
+int mbedtls_pk_setup_opaque( mbedtls_pk_context *ctx, const psa_key_slot_t key )
 {
-    const mbedtls_pk_info_t * const info = &mbedtls_pk_opaque_psa_info;
+    const mbedtls_pk_info_t * const info = &mbedtls_pk_opaque_info;
     psa_key_slot_t *pk_ctx;
     psa_key_type_t type;