dt-binding: i2c: add bus-supply property

In some platforms, they disable the power-supply of i2c due
to power consumption reduction. This patch add bus-supply property.
Signed-off-by: Bibby Hsieh <bibby.hsieh@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
[wsa: rebased to i2c/for-next]
Signed-off-by: Wolfram Sang <wsa@kernel.org>

Documentation/devicetree/bindings/i2c/i2c.txt

@@ -23,6 +23,9 @@ Optional properties (per bus)
 These properties may not be supported by all drivers. However, if a driver
 wants to support one of the below features, it should adapt these bindings.
 
+- bus-supply
+	phandle to the regulator that provides power to SCL/SDA.
+
 - clock-frequency
 	frequency of bus clock in Hz.