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>

arch/arm/lib/semihosting.c

@@ -191,7 +191,7 @@ static int do_smhload(struct cmd_tbl *cmdtp, int flag, int argc,
 		int ret;
 		char end_str[64];
 
-		load_addr = simple_strtoul(argv[2], NULL, 16);
+		load_addr = hextoul(argv[2], NULL);
 		if (!load_addr)
 			return -1;