extmod/modopenamp: Fix Endpoint callback required arg.

The callback arg is not actually required.
Signed-off-by: iabdalkader <i.abdalkader@gmail.com>

extmod/modopenamp.c

@@ -221,7 +221,7 @@ static mp_obj_t endpoint_make_new(const mp_obj_type_t *type, size_t n_args, size
     enum { ARG_name, ARG_callback, ARG_src, ARG_dest };
     static const mp_arg_t allowed_args[] = {
         { MP_QSTR_name, MP_ARG_OBJ | MP_ARG_REQUIRED,  {.u_rom_obj = MP_ROM_NONE } },
-        { MP_QSTR_callback, MP_ARG_OBJ | MP_ARG_REQUIRED, {.u_rom_obj = MP_ROM_NONE } },
+        { MP_QSTR_callback, MP_ARG_OBJ, {.u_rom_obj = MP_ROM_NONE } },
         { MP_QSTR_src, MP_ARG_INT | MP_ARG_KW_ONLY,  {.u_int = RPMSG_ADDR_ANY } },
         { MP_QSTR_dest, MP_ARG_INT | MP_ARG_KW_ONLY,  {.u_int = RPMSG_ADDR_ANY } },
     };