pinctrl: fix example .get_group_pins implementation signature

The callback function signature has changed in commit a5818a8b (pinctrl:
get_group_pins() const fixes)

Fixes: a5818a8b ('pinctrl: get_group_pins() const fixes')
Cc: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>