tty/serial: add support for Xilinx PS UART

The Xilinx PS Uart is used on the new ARM based SoC. This
UART is not compatible with others such that a seperate
driver is required.

Signed-off-by: John Linn <john.linn@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>