ARM: dts: imx6qdl-sabresd: Add an mdio node

ARM: dts: imx6qdl-sabresd: Add an mdio node

imx6qdl-sabresd has an Atheros AR8031 Ethernet PHY at address 1.

The AR8031 provides a 125MHz clock to the ENET_REF_CLK i.MX6 pin.

Improve the Ethernet representation by adding an mdio node with such
information.

An advantage of adding the mdio node is that the AR8031 initialization
code in the mx6sabresd board file in U-Boot can totally be removed.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Soeren Moch <smoch@web.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>