extmod/machine_pinbase: Put PinBase singleton in ROM.

This patch also removes the empty type "pinbase_type" (which crashes if
accessed) and uses "machine_pinbase_type" instead as the type of the
PinBase singleton.

Author: dpgeorge

extmod/machine_pinbase.c

@@ -30,6 +30,7 @@
 #include "py/obj.h"
 #include "py/runtime.h"
 #include "extmod/virtpin.h"
+#include "extmod/machine_pinbase.h"
 
 // PinBase class
 
@@ -40,10 +41,8 @@ typedef struct _mp_pinbase_t {
     mp_obj_base_t base;
 } mp_pinbase_t;
 
-STATIC const mp_obj_type_t pinbase_type;
-
-STATIC mp_pinbase_t pinbase_singleton = {
-    .base = { &pinbase_type },
+STATIC const mp_pinbase_t pinbase_singleton = {
+    .base = { &machine_pinbase_type },
 };
 
 STATIC mp_obj_t pinbase_make_new(const mp_obj_type_t *type, size_t n_args, size_t n_kw, const mp_obj_t *args) {