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/remoteproc.c b/cmd/remoteproc.c
index b3ddceb..2a0e575 100644
--- a/cmd/remoteproc.c
+++ b/cmd/remoteproc.c
@@ -130,9 +130,9 @@
 		return CMD_RET_USAGE;
 
 	id = (int)simple_strtoul(argv[1], NULL, 10);
-	addr = simple_strtoul(argv[2], NULL, 16);
+	addr = hextoul(argv[2], NULL);
 
-	size = simple_strtoul(argv[3], NULL, 16);
+	size = hextoul(argv[3], NULL);
 
 	if (!size) {
 		printf("\t Expect some size??\n");