iio: core: Introduce IIO_CHAN_INFO_OVERSAMPLING_RATIO

Some magnetometers can perform a number of repetitions in HW
for each measurement to increase accuracy. One example is
Bosch BMC150:
http://ae-bst.resource.bosch.com/media/products/dokumente/bmc150/BST-BMC150-DS000-04.pdf.

Introduce an interface to set the oversampling ratio
for these devices.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

diff --cc Documentation/ABI/testing/sysfs-bus-iio
Simple merge

diff --cc drivers/iio/industrialio-core.c
Simple merge

diff --cc include/linux/iio/iio.h
Simple merge