dt-bindings: iio: accel: Add required regulators to bma180

The bma180 and related chips should have two regulators attached to
them.  The IIO driver currently uses them, document them here as
well.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Bakker <xc-racer2@live.ca>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>