iio: support oversampling_ratio configuration in iio node

[laykuanloon]

@peijiajames

BMP280 iio driver contains "oversampling_ratio", can support the configuration from iio node ? For e.g: by setting "oversampling_ration" in json file.

---

What: /sys/bus/iio/devices/iio:deviceX/in_magn_x_oversampling_ratio
What: /sys/bus/iio/devices/iio:deviceX/in_magn_y_oversampling_ratio
What: /sys/bus/iio/devices/iio:deviceX/in_magn_z_oversampling_ratio
KernelVersion: 4.2
Contact: linux-iio@vger.kernel.org
Description:
Hardware applied number of measurements for acquiring one
data point. The HW will do [_name]_oversampling_ratio
measurements and return the average value as output data. Each
value resulted from [_name]_oversampling_ratio measurements
is considered as one sample for [_name]_sampling_frequency.

@elvinongbl FYI

[joyceooi295]

@edersondisouza @ceolin @bdilly @barbieri Please help

[peijiajames]

@guchaojie is working on this already. @guchaojie please comment with the implementation.

[guchaojie]

@joyceooi295 We would add this attribute into json file for user to set in iio node just like other attribute support method

[yongli3]

@laykuanloon @peijiajames @guchaojie Based on kernel commit id 62979904b0037430ecc7d8b682f684adced1340f. It seems to me that only kernel 4.8+ supports this "oversampling" feature for BMP280