uart: native_posix: Add support for another serial port

The second serial port (UART_1) can be used for connecting to
host serial port. This is used for example by PPP (Point-to-Point
Protocol) implementation in which case the pppd running in Linux host
connects to a pty that is linked to UART_1 in Zephyr.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>