pinctrl: mxs: Use kfree to fix build error

commit 0bf74818 (pinctrl: pinctrl-mxs: Take care of frees if the kzalloc fails)
introduced the following build error:

drivers/pinctrl/pinctrl-mxs.c:140:3: error: implicit declaration of function 'free'

Use kfree function instead.
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Devendra Naga <devendra.aaru@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/pinctrl-mxs.c

@@ -137,7 +137,7 @@ static int mxs_dt_node_to_map(struct pinctrl_dev *pctldev,
 
 free_group:
 	if (!purecfg)
-		free(group);
+		kfree(group);
 free:
 	kfree(new_map);
 	return ret;