ARM: dts: iwg23s-sbc: specify EtherAVB PHY IRQ

Specify  EtherAVB PHY IRQ  in the board specific device tree, now that we
have GPIO support.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>