serial: 8250_fintek: Add F81966 Support

Fintek F81966 is a LPC/eSPI to 6 UARTs SuperIO. It has fully compatible
with F81866. It's also need check the IRQ mode with system assigned.

F81966 IRQ Mode setting:
        0xf0
                Bit1: IRQ_MODE0
                Bit0: Share mode (always on)
        0xf6
                Bit3: IRQ_MODE1

        Level/Low: IRQ_MODE0:0, IRQ_MODE1:0
        Edge/High: IRQ_MODE0:1, IRQ_MODE1:0

Signed-off-by: Ji-Ze Hong (Peter Hong) <hpeter+linux_kernel@gmail.com>
Cc: Ji-Ze Hong (Peter Hong) <peter_hong@fintek.com.tw>
Link: https://lore.kernel.org/r/20200528022429.32078-1-hpeter+linux_kernel@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>