gpio: acpi: Split out acpi_gpio_get_irq_resource() helper

The helper does retrieve pointer to struct acpi_resource_gpio from
struct acpi_resource if it represents GpioInt() resource.

It will be used by PNP code later on.

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