tty: xuartps: Make driver depend on ARCH_ZYNQ

This is a driver for the UART of Xilinx' Zynq and should depend on this
architecture.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>