pinctrl: sirf: initialize the irq_chip pointer of pinctrl_gpio_range

This patch initializes the optional irq_chip pointer gc in sirfsoc
pinctrl_gpio_range.

Signed-off-by: Baohua Song <Baohua.Song@csr.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>