pinctrl: iproc: Use platform_get_irq_optional() to avoid error message

Use platform_get_irq_optional() instead of platform_get_irq() to avoid
below error message during probe:

[ 0.589121] iproc-gpio 66424800.gpio: IRQ index 0 not found

Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Link: https://lore.kernel.org/r/20200110035524.23511-1-rayagonda.kokatanur@broadcom.com


Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>