serial: 8250_fintek: Add F81865 Support

Fintek F81865 is a LPC to 6 UARTs SuperIO. It has less functional UARTs
likes F81866. It's also need check the IRQ mode with system assigned,
but the configuration is not the same with F81216 series.

F81865 IRQ Mode setting:
    0xf0
            Bit1: IRQ_MODE0
            Bit0: Share mode (always on)

    Level/Low: IRQ_MODE0:0
    Edge/High: IRQ_MODE0:1

The following list is brief descriptions of F81865:

F81865 (0704)
    9Bit(not implements with mainline)
    RS485(implemented)

Suggested-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Ji-Ze Hong (Peter Hong) <hpeter+linux_kernel@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>