global: Convert simple_strtoul() with decimal to dectoul()

It is a pain to have to specify the value 10 in each call. Add a new
dectoul() function and update the code to use it.

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

cmd/led.c

@@ -93,7 +93,7 @@ int do_led(struct cmd_tbl *cmdtp, int flag, int argc, char *const argv[])
 	if (cmd == LEDST_BLINK) {
 		if (argc < 4)
 			return CMD_RET_USAGE;
-		freq_ms = simple_strtoul(argv[3], NULL, 10);
+		freq_ms = dectoul(argv[3], NULL);
 	}
 #endif
 	ret = led_get_by_label(led_label, &dev);