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/io.c b/cmd/io.c
index c7e9641..e23ea62 100644
--- a/cmd/io.c
+++ b/cmd/io.c
@@ -51,7 +51,7 @@
 			return 1;
 
 		/* Address is specified since argc > 1 */
-		addr = simple_strtoul(argv[1], NULL, 16);
+		addr = hextoul(argv[1], NULL);
 		addr += base_address;
 
 		/*
@@ -59,7 +59,7 @@
 		 * Length is the number of objects, not number of bytes.
 		 */
 		if (argc > 2)
-			length = simple_strtoul(argv[2], NULL, 16);
+			length = hextoul(argv[2], NULL);
 	}
 
 	bytes = size * length;
@@ -102,8 +102,8 @@
 	if (size < 0)
 		return 1;
 
-	addr = simple_strtoul(argv[1], NULL, 16);
-	val = simple_strtoul(argv[2], NULL, 16);
+	addr = hextoul(argv[1], NULL);
+	val = hextoul(argv[2], NULL);
 
 	if (size == 4)
 		outl((u32) val, addr);