dt-bindings: rtc: ti,bq32k: Convert to json-schema

Convert the TI BQ32000 I2C Serial Real-Time Clock Device Tree binding
documentation to json-schema.

Document missing properties.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/42d9c71b4ee1f120e0cdcf6b266547d29d1fb9a4.1623851377.git.geert+renesas@glider.be