dt-bindings: clock: Convert marvell,mmp2-clock to json-schema

dt-bindings: clock: Convert marvell,mmp2-clock to json-schema

Convert the fixed-factor-clock binding to DT schema format using
json-schema.

While at that, fix a couple of small errors: make the file base name
match the compatible string, add an example and document the reg-names
property.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lkml.kernel.org/r/20200309194254.29009-4-lkundrak@v3.sk
Signed-off-by: Stephen Boyd <sboyd@kernel.org>