dt-bindings: net: xilinx: document xilinx emaclite driver binding

Add basic description for the xilinx emaclite driver DT bindings.

Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>