gpio: mvebu: avoid error message for optional IRQ

platform_get_irq() will generate an error message if the requested irq
is not present

  mvebu-gpio f1010140.gpio: IRQ index 3 not found

use platform_get_irq_optional() to avoid the error message being
generated.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>