dt-bindings: net: xilinx_gmii2rgmii: Convert to json schema

Convert the Xilinx GMII to RGMII Converter device tree binding
documentation to json schema.
This converter is usually used as gem <---> gmii2rgmii <---> external phy
and, it's phy-handle should point to the phandle of the external phy.

Signed-off-by: Pranavi Somisetty <pranavi.somisetty@amd.com>
Signed-off-by: Harini Katakam <harini.katakam@amd.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>