ARM: dts: iwg20d-q7: Add Ethernet AVB support

Define the iWave RainboW-G20D-Qseven board dependent part of the Ethernet
AVB device node.

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