dt-bindings: net: add bindings for ADIN PHY driver

This change adds bindings for the Analog Devices ADIN PHY driver, detailing
all the properties implemented by the driver.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>