dt-bindings: net: stmmac: Convert the binding to a schemas

Switch the STMMAC / Synopsys DesignWare MAC controller binding to a YAML
schema to enable the DT validation.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>