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

Documentation/devicetree/bindings/rtc/rtc.yaml

@@ -52,6 +52,12 @@ properties:
       Selected resistor for trickle charger. Should be given
       if trickle charger should be enabled.
 
+  trickle-voltage-millivolt:
+    description:
+      Selected voltage for trickle charger. Should be given
+      if trickle charger should be enabled and the trickle voltage is different
+      from the RTC main power supply.
+
   wakeup-source:
     $ref: /schemas/types.yaml#/definitions/flag
     description: