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/mvebu/comphy_rx_training.c b/cmd/mvebu/comphy_rx_training.c
index 0798dec..25a9e15 100644
--- a/cmd/mvebu/comphy_rx_training.c
+++ b/cmd/mvebu/comphy_rx_training.c
@@ -25,8 +25,8 @@
 		return -1;
 	}
 
-	cp_index = simple_strtoul(argv[1], NULL, 16);
-	comphy_index = simple_strtoul(argv[2], NULL, 16);
+	cp_index = hextoul(argv[1], NULL);
+	comphy_index = hextoul(argv[2], NULL);
 
 	ret = uclass_get(UCLASS_MISC, &uc);
 	if (ret) {