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>