dt-bindings: iio: gyroscope: bmg160: Document regulator supplies

BMG160 needs VDD and VDDIO regulators that might need to be explicitly
enabled. Document support for vdd/vddio-supply to implement this.

Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201211183815.51269-1-stephan@gerhold.net


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>