tty: serial: imx: Use platform_get_irq_optional() for optional IRQs

tty: serial: imx: Use platform_get_irq_optional() for optional IRQs

All i.MX SoCs except i.MX1 have ONLY one necessary IRQ, use
platform_get_irq_optional() to get second/third IRQ which are
optional to avoid below error message during probe:

[    0.726219] imx-uart 30860000.serial: IRQ index 1 not found
[    0.731329] imx-uart 30860000.serial: IRQ index 2 not found

Fixes: 682a742ed79d525d ("driver core: platform: Add an error message to platform_get_irq*()")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Link: https://lore.kernel.org/r/1570614559-11900-1-git-send-email-Anson.Huang@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>