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/mdio.c b/cmd/mdio.c
index cfa45ad..3c74326 100644
--- a/cmd/mdio.c
+++ b/cmd/mdio.c
@@ -254,7 +254,7 @@
 	switch (op[0]) {
 	case 'w':
 		if (pos > 1)
-			data = simple_strtoul(argv[pos--], NULL, 16);
+			data = hextoul(argv[pos--], NULL);
 		/* Intentional fall-through - Get reg for read and write */
 	case 'r':
 		if (pos > 1)