gpio: mockup: refactor the module init function

Let's move the code preparing the device properties into a separate
routine. This has the advantage of simplifying the error handling and
makes the indentation less deep.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>