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/sleep.c b/cmd/sleep.c
index 1fff400..c741b4a 100644
--- a/cmd/sleep.c
+++ b/cmd/sleep.c
@@ -20,7 +20,7 @@
 	if (argc != 2)
 		return CMD_RET_USAGE;
 
-	delay = simple_strtoul(argv[1], NULL, 10) * CONFIG_SYS_HZ;
+	delay = dectoul(argv[1], NULL) * CONFIG_SYS_HZ;
 
 	frpart = strchr(argv[1], '.');