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/arch/arm/mach-imx/imx8/ahab.c b/arch/arm/mach-imx/imx8/ahab.c
index 6392fe2..015267c 100644
--- a/arch/arm/mach-imx/imx8/ahab.c
+++ b/arch/arm/mach-imx/imx8/ahab.c
@@ -161,7 +161,7 @@
 	if (argc < 2)
 		return CMD_RET_USAGE;
 
-	addr = simple_strtoul(argv[1], NULL, 16);
+	addr = hextoul(argv[1], NULL);
 
 	printf("Authenticate OS container at 0x%lx\n", addr);