dt-bindings: rtc: microcrystal,rv3032: Add missing type to 'trickle-voltage-millivolt'

'trickle-voltage-millivolt' is missing a type definition. '-millivolt' is
not a standard unit (should be '-microvolt'). As the property is already
in use, add a type reference.

Signed-off-by: Rob Herring <[email protected]>
Signed-off-by: Alexandre Belloni <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: robherring

Documentation/devicetree/bindings/rtc/microcrystal,rv3032.yaml

@@ -32,6 +32,7 @@ properties:
       - 11000
 
   trickle-voltage-millivolt:
+    $ref: /schemas/types.yaml#/definitions/uint32
     enum:
       - 1750
       - 3000