gpio: mockup: increase the number of supported device properties

The driver actually supports 4 properties but we only ever set up up to
three. This will change however in upcoming patches so increase the
number of really (as in: the number the property array can hold)
supported properties to 4.

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