gpiolib: acpi: Add acpi_gpio_get_io_resource()

Add a function to verify that a given ACPI resource represents a GpioIo()
type of resource, and return it if so.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Daniel Scally <djrscally@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>