drivers: i2c: add Xilinx AXI I2C driver

Add a driver to support the Xilinx AXI IIC Bus Interface logic core, as
described in Xilinx document PG090. This can be configured for use on
most Xilinx FPGA-based platforms such as the Digilent Arty.

Both the 2.00a and 2.1 versions of the core are supported. The 2.00a
revision has a bug causing dynamic read mode to not be reliable. With
this version of the core the driver falls back to reading only 1 byte
per interrupt.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>