dt-bindings: net: convert mscc-miim to YAML format

Convert the mscc-miim device tree binding to the new YAML format.

The original binding don't mention if the interrupt property is optional
or not. But on the SparX-5 SoC, for example, the interrupt property isn't
used, thus in the new binding that property is optional. FWIW the driver
doesn't use interrupts at all.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>