gpio: pxa: Avoid a warning when gpio0 and gpio1 IRQS are not there

Not all platforms use those. Let's use
platform_get_irq_byname_optional() instead platform_get_irq_byname() so
that we avoid a useless warning:

  [    1.359455] pxa-gpio d4019000.gpio: IRQ gpio0 not found
  [    1.359583] pxa-gpio d4019000.gpio: IRQ gpio1 not found

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>