dt-bindings: iio: accel: mma8452: add power supplies property

The accelerometer's power supplies could be controllable on some
platforms, add property "vdd/vddio" power supply to let device tree
to pass phandles to the regulators to driver.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>