iio: pressure: bmp280: support supply regulators

The BMP085/BMP180/BMP280 is supplied with two power sources:
VDDA (analog power) and VDDD (digital power). As these may come
from regulators (as on the APQ8060 Dragonboard) we need the driver
to attempt to fetch and enable these regulators.

We FAIL if we cannot: boards should either define:
- Proper regulators if present
- Define fixed regulators if power is hardwired to the component
- Rely on dummy regulators (will be present on all DT systems and
  any boardfile system that calls regulator_has_full_constraints().

Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>