dt-bindings: iio: humidity: hts221: introduce vdd regulator bindings

Signed-off-by: NLorenzo Bianconi <lorenzo@kernel.org>
Link: https://lore.kernel.org/r/9579804f52ccab74a5ca5c7a55b5072b7304bea9.1606045688.git.lorenzo@kernel.orgSigned-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/devicetree/bindings/iio/humidity/st,hts221.yaml

@@ -26,6 +26,8 @@ properties:
       The interrupt/data ready line will be configured as open drain, which
       is useful if several sensors share the same interrupt line.
 
+  vdd-supply: true
+
   interrupts:
     maxItems: 1