dt/bindings: remove users of device_type "mdio"

device_type is a deprecated property, but some MDIO bus nodes still have
it. Except for a couple of old binding (compatible="gianfar" and
compatible="ucc_geth_phy") the kernel doesn't look for
device_type="mdio" at all.

This patch removes all instances of device_type="mdio" from the binding
documentation and the .dts files.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/boot/dts/mpc7448hpc2.dts

@@ -68,7 +68,6 @@
 		};
 
 		MDIO: mdio@6000 {
-			device_type = "mdio";
 			compatible = "tsi108-mdio";
 			reg = <0x6000 0x50>;
 			#address-cells = <1>;