iio: accel: BMC150 accel support

iio: accel: BMC150 accel support

This change implements BMC150 accelerometer driver. A BMC150 package
consist of a compass and an accelerometer. This driver only implements
accelerometer part.
Spec downloaded from:
http://ae-bst.resource.bosch.com/media/products/dokumente/bmc150/BST-BMC150-DS000-03.pdf

This sensor chip supports many advanced features, but this driver
implements minimum feature set which is a must to be useful.
This driver can be enhanced incrementally.
If the sensor vendor wants to update full featured version, they
can substitute or enhance this driver when they get chance.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>