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

The networking PHYs have a number of available device tree properties that
can be used in their device tree node. Add a YAML schemas for those.

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