dt-bindings: net: xilinx_gmii2rgmii: Add clock support

Add "clocks" bindings for the input clock.

Signed-off-by: Vineeth Karumanchi <vineeth.karumanchi@amd.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>