iio: add bmp280 pressure and temperature driver

Minimal implementation, can read temperature and data using direct mode.

Datasheet available at
<http://ae-bst.resource.bosch.com/media/products/dokumente/bmp280/BST-BMP280-DS001-09.pdf

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Signed-off-by: Vlad Dogaru <vlad.dogaru@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>