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/drivers/video/mx3fb.c b/drivers/video/mx3fb.c
index 9b42ca8..e6dd2b8 100644
--- a/drivers/video/mx3fb.c
+++ b/drivers/video/mx3fb.c
@@ -820,7 +820,7 @@
 	if (penv) {
 		/* decide if it is a string */
 		if (penv[0] <= '9') {
-			videomode = (int) simple_strtoul(penv, NULL, 16);
+			videomode = (int)hextoul(penv, NULL);
 			tmp = 1;
 		}
 	} else {