iio: magn: Add support for BMM150 magnetometer

BMM150 is register compatible with magnetometer part of
BMC156.

Datasheet is at:
http://www.mouser.com/ds/2/783/BST-BMM150-DS001-01-786480.pdf



Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>