i2c: cadence: Added slave support

Added support for I2C slave functionality
Signed-off-by: Chirag Parekh <chirag.parekh@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Radu Pirea <radu_nicolae.pirea@upb.ro>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>