dt-bindings: timer: Convert mrvl,mmp-timer to json-schema

A straightforward conversion of the mrvl,mmp-timer binding to DT schema
format using json-schema.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20200616223353.993567-6-lkundrak@v3.sk


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