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>