ARM: dts: imx6dl-riotboard: properly define rgmii PHY

The Atheros AR8035 PHY can be autodetected but can't use interrupt
support provided on this board. Define MDIO bus and the PHY node to make
it work properly.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>