gpio: pca935x: Allow IRQ support for driver built as a module

Perhaps by some historical reasons the IRQ support has been allowed
only for built-in driver. However, there is nothing prevents us
to build it as module an use as IRQ chip.

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