dt-bindings: phy: Convert phy-mmp3-usb to json-schema

A rather straightforward conversion of the phy-mmp3-usb binding to DT
schema format using json-schema.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
[robh: add additionalProperties]
Signed-off-by: Rob Herring <robh@kernel.org>