global: Convert simple_strtoul() with hex to hextoul()

It is a pain to have to specify the value 16 in each call. Add a new
hextoul() function and update the code to use it.

Add a proper comment to simple_strtoul() while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/demo.c b/cmd/demo.c
index 78a55f7..a2957f7 100644
--- a/cmd/demo.c
+++ b/cmd/demo.c
@@ -48,7 +48,7 @@
 	int ret;
 
 	if (argc) {
-		light = simple_strtoul(argv[0], NULL, 16);
+		light = hextoul(argv[0], NULL);
 		ret = demo_set_light(demo_dev, light);
 	} else {
 		ret = demo_get_light(demo_dev);