iio: pressure: bmp280: support device tree initialization

This adds device tree support to the BMP085, BMP180 and BMP280
pressure sensors. Tested on the Qualcomm APQ8060 Dragonboard:

iio:device1$ cat in_temp_input
26700
iio:device1$ cat in_pressure_input
99.185000000

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