serial/8250_fintek: Support for multiple base_ports

Fintek chip can be connected at address 0x4e and also 0x2e.
Add some logic to find out the address of the chip.

Reported-by: Peter Hong <peter_hong@fintek.com.tw>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>