dt-bindings: net: Add YAML schemas for the generic Ethernet options

The Ethernet controllers have a good number of generic options that can be
needed in a device tree. Add a YAML schemas for those.

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