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/lib/dhry/cmd_dhry.c b/lib/dhry/cmd_dhry.c
index d55ab54..77b52a2 100644
--- a/lib/dhry/cmd_dhry.c
+++ b/lib/dhry/cmd_dhry.c
@@ -16,7 +16,7 @@
 	int iterations = 1000000;
 
 	if (argc > 1)
-		iterations = simple_strtoul(argv[1], NULL, 10);
+		iterations = dectoul(argv[1], NULL);
 
 	start = get_timer(0);
 	dhry(iterations);