pinctrl: baytrail: Register GPIO chip after chip->to_irq is set

If chip->to_irq is NULL ACPI GPIO helpers don't register GPIO event
handlers thus preventing any ACPI GPIO triggered events. Solve this by
calling gpiochip_add() after we have set up drivers chip->to_irq hook.

Signed-off-by: Jin Yao <yao.jin@intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>