platform/chrome: cros_ec_lpc: Use platform_get_irq_optional() for optional IRQs

As platform_get_irq() now prints an error when the interrupt does not
exist, use platform_get_irq_optional() to get the IRQ which is optional
to avoid below error message during probe:

  [    5.113502] cros_ec_lpcs GOOG0004:00: IRQ index 0 not found

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>