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>
diff --git a/cmd/legacy_led.c b/cmd/legacy_led.c
index 86cd969..5256255 100644
--- a/cmd/legacy_led.c
+++ b/cmd/legacy_led.c
@@ -129,7 +129,7 @@
 				if (argc != 4)
 					return CMD_RET_USAGE;
 
-				freq = simple_strtoul(argv[3], NULL, 10);
+				freq = dectoul(argv[3], NULL);
 				__led_blink(led_commands[i].mask, freq);
 			}
 			/* Need to set only 1 led if led_name wasn't 'all' */