drivers: serial: uart_xlnx_ps: switch over to DEVICE_MMIO API

Switch the Xilinx PS UART device driver over to the use of the
DEVICE_MMIO_... macros instead of just using the physical base
address from the device tree.

Signed-off-by: Immo Birnbaum <Immo.Birnbaum@weidmueller.com>