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>
diff --git a/drivers/serial/serial.c b/drivers/serial/serial.c
index dea25a0..ebbd219 100644
--- a/drivers/serial/serial.c
+++ b/drivers/serial/serial.c
@@ -61,7 +61,7 @@
 		/*
 		 * Switch to new baudrate if new baudrate is supported
 		 */
-		baudrate = simple_strtoul(value, NULL, 10);
+		baudrate = dectoul(value, NULL);
 
 		/* Not actually changing */
 		if (gd->baudrate == baudrate)