dt-bindings: rtc: ds1307: Convert to json-schema

Convert the DS1307 (and compatible) RTC bindings from the free-form text
format to json-schema.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220617114420.1398259-1-thierry.reding@gmail.com