pinctrl: fix example .get_group_pins implementation signature

The callback function signature has changed in commit a5818a8bd0 (pinctrl:
get_group_pins() const fixes)

Fixes: a5818a8bd0 ('pinctrl: get_group_pins() const fixes')
Cc: Stephen Warren <[e-mail]>
Signed-off-by: Baruch Siach <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

Documentation/pinctrl.txt

···
 }
 
 static int foo_get_group_pins(struct pinctrl_dev *pctldev, unsigned selector,
-			       unsigned ** const pins,
-			       unsigned * const num_pins)
+			       const unsigned **pins,
+			       unsigned *num_pins)
 {
 	*pins = (unsigned *) foo_groups[selector].pins;
 	*num_pins = foo_groups[selector].num_pins;