dt-bindings: rtc: add trickle-voltage-millivolt

Some RTCs have a trickle charge that is able to output different voltages
depending on the type of the connected auxiliary power (battery, supercap,
...). Add a property allowing to specify the necessary voltage.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201013144110.1942218-1-alexandre.belloni@bootlin.com