drivers: i2c: i2c_xilinx_axi: switch to DEVICE_MMIO API

Switch the Xilinx AXI I2C device driver to use the DEVICE_MMIO_...
macros instead of directly using the physical base address from the
device tree. This allows the driver to work on SoCs with an MMU,
such as the Zynq-7000.

Signed-off-by: Simon Maurer <mail@maurer.systems>

Author: mausys