The interrupt is not required so use platform_irq_get_optional() to
avoid error messages like

  i2c-pca-platform 22080000.i2c: IRQ index 0 not found
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>