dt-bindings: net: Document moca PHY interface

MoCA (Multimedia over Coaxial) is used by the internal GENET/MOCA cores
and will be needed in order to convert GENET to YAML in subsequent
changes.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20211206180049.2086907-4-f.fainelli@gmail.com


Signed-off-by: Rob Herring <robh@kernel.org>