dt-bindings: phy: Convert Calxeda ComboPHY binding to json-schema

Convert the Calxeda ComboPHY binding to DT schema format using
json-schema.
There is no driver in the Linux kernel matching the compatible
string, but the nodes are parsed by the SATA driver, which links to them
using its port-phys property.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>