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