gpiolib: Get rid of gpio_bus_match() forward declaration

There is nothing specific about gpio_bus_match(), so we may
simply move it to the top of the file and get rid of forward
declaration.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>