ARM: dts: iwg23s-sbc: Add EtherAVB support

Define the iW-RainboW-G23S board dependent part of the
EtherAVB device node.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Chris Paterson <chris.paterson2@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>