serial: uartps: Move cnds_uart_get_port to probe

c&p this function to probe as preparation for removing
cdns_uart_port[] static array.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>