ARM: dts: imx6q-tbs2910: Add an mdio node

imx6q-tbs2910 has an Atheros AR8035 Ethernet PHY at address 4.

The AR8035 provides a 125MHz clock to the ENET_REF_CLK i.MX6 pin.

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

This fixes an Ethernet regression in U-Boot as U-Boot AR803X driver now
expects the 'qca,clk-out-frequency' property to be passed via
device tree.

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