i2c: Add support for Xilinx XPS IIC Bus Interface

This patch adds support for the Xilinx XPS IIC Bus Interface.

The driver uses the dynamic mode, supporting to put several
I2C messages in the FIFO to reduce the number of interrupts.

It has the same feature as ocores, it can be passed a list
of devices that will be added when the bus is probed.

Signed-off-by: Richard Röjfors <richard.rojfors@pelagicore.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>