gpio: merrifield: Move hardware initialization to callback

The driver wants to initialize related registers before IRQ chip will be added.
That's why move it to a corresponding callback. It also fixes the NULL pointer
dereference.

Fixes: 8f86a5b4 ("gpio: merrifield: Pass irqchip when adding gpiochip")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>