dt-bindings: rtc: isl1208: Convert to json-schema

Convert the isl1208 RTC device tree binding documentation to json-schema.

Update the example to match reality.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230623140948.384762-5-biju.das.jz@bp.renesas.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>