powerpc/mpc8313erdb: fix kernel panic because mdio device is not probed

Probe the new mdio node added by b31a1d8b. Fix kernel panic problem when
gianfar driver wants to get the of_platform_device of that mdio.

Signed-off-by: Li Yang <[e-mail]>
Signed-off-by: Kumar Gala <[e-mail]>

arch/powerpc/boot/dts/mpc8313erdb.dts

···
 			cell-index = <0>;
 			device_type = "network";
 			model = "eTSEC";
-			compatible = "gianfar", "simple-bus";
+			compatible = "gianfar";
 			reg = <0x24000 0x1000>;
 			local-mac-address = [ 00 00 00 00 00 00 ];
 			interrupts = <37 0x8 36 0x8 35 0x8>;

arch/powerpc/platforms/83xx/mpc831x_rdb.c

···
 
 static struct of_device_id __initdata of_bus_ids[] = {
 	{ .compatible = "simple-bus" },
+	{ .compatible = "gianfar" },
 	{},
 };