dt-bindings: rtc: rv3032: add RV-3032 bindings

Document the Microcrystal RV-3032 device tree bindings

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201013144110.1942218-2-alexandre.belloni@bootlin.com