gpio: Warn about invalid GPIOs used with the 'gpio' command

At present there is no indication that an invalid GPIO is used except that
the GPIO status is not displayed. Make the error more explicit to avoid
confusion.

Signed-off-by: Simon Glass <sjg@chromium.org>

common/cmd_gpio.c

@@ -174,8 +174,10 @@ static int do_gpio(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 	 * GPIO compatibility layer.
 	 */
 	ret = gpio_lookup_name(str_gpio, NULL, NULL, &gpio);
-	if (ret)
+	if (ret) {
+		printf("GPIO: '%s' not found\n", str_gpio);
 		return cmd_process_error(cmdtp, ret);
+	}
 #else
 	/* turn the gpio name into a gpio number */
 	gpio = name_to_gpio(str_gpio);