Documentation: DT: net: Add Xilinx gmiitorgmii converter device tree binding documentation

Device-tree binding documentation for xilinx gmiitorgmii converter.

Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>