gpio: altera: change to platform_get_irq_optional to avoid false-positive error

This patch switches to platform_get_irq_optional() from
platform_get_irq() as it causes a false-positive error such as 'IRQ
index 0 not found' when IRQ is not used. The IRQ usage is optional in
this gpio-altera driver, so the error log is undesirable.

Signed-off-by: Ooi, Joyce <joyce.ooi@intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>