pinctrl/sirf: fix sirfsoc_get_group_pins prototype

fix sirfsoc_get_group_pins prototype introduced in 7e570f97, we
missed to de-constify a pointer.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>