iio: pressure: bmp280: augment DT bindings

This adds standard device tree bindings for a reset GPIO line, and
the VDDD and VDDA power regulators.

Cc: devicetree@vger.kernel.org
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>