pinctrl: lynxpoint: Use platform_get_irq_optional() explicitly

There is no need to repeat functionality of platform_get_irq_optional()
in the driver. Replace it with explicit call to the helper.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>