iio: magn: bmc150_magn: add oversampling ratio

Export the oversampling ratio so that the user can change the
number of repetions for x/y/z axis.

The sampling frequency is limited by the oversampling ratio.
The available sampling frequencies might change depending
on the values of oversampling_ratio.

The specification can be downloaded from:
http://ae-bst.resource.bosch.com/media/products/dokumente/bmc150/BST-BMC150-DS000-04.pdf.

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