iio: pressure: bmp280: add ability to control oversampling rate

This adds ability to control the oversampling ratio of the temperature
and pressure measurement for both bmp180 and bmp280.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Vlad Dogaru <vlad.dogaru@intel.com>
Cc: Christoph Mair <christoph.mair@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>