dt-bindings: i2c: xiic: Add 'xlnx,axi-iic-2.1' to compatible

Add xilinx I2C new version 'xlnx,axi-iic-2.1' string to compatible.
Add clock-frequency as optional property.

Signed-off-by: Raviteja Narayanam <raviteja.narayanam@xilinx.com>
Signed-off-by: Manikanta Guntupalli <manikanta.guntupalli@amd.com>
Acked-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>