dt-bindings: i2c: Convert i2c-pxa to json-schema

dt-bindings: i2c: Convert i2c-pxa to json-schema

A conversion of the i2c-pxa binding to DT schema format using json-schema.

This also cleans ups some errors in the binding: The compatible string
description suggested that "mmp" in "mrvl,mmp-twsi" is to be substituted
with a processor model, which wouldn't be a right thing to do and indeed
nobody seems to have been doing that. There also was "Recommended
properties" section that included optional as well as mandatory
properties. Missing mandatory properties were added to the example.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20200616223353.993567-3-lkundrak@v3.sk
Signed-off-by: Rob Herring <robh@kernel.org>