dt-bindings: net: renesas,etheravb: Convert to json-schema

Convert the Renesas Ethernet AVB (EthernetAVB-IF) Device Tree binding
documentation to json-schema.

Add missing properties.
Update the example to match reality.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>