global: Convert simple_strtoul() with decimal to dectoul()

It is a pain to have to specify the value 10 in each call. Add a new
dectoul() function and update the code to use it.

Signed-off-by: Simon Glass <sjg@chromium.org>

cmd/clk.c

@@ -120,7 +120,7 @@ static int do_clk_setfreq(struct cmd_tbl *cmdtp, int flag, int argc,
 	s32 freq;
 	struct udevice *dev;
 
-	freq = simple_strtoul(argv[2], NULL, 10);
+	freq = dectoul(argv[2], NULL);
 
 	dev = clk_lookup(argv[1]);