gpio: merrifield: Protect irq_ack() and gpio_set() by lock

There is a potential race when two threads do the writes to the same register
in parallel.

Prevent out of order in such case by protecting I/O access by spin lock.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>