iio:gyro:bmg160 Gyro Sensor driver

This change implements support for BMG160 Gyro sensor. Although chip
has several advanced features, this change implements minimum set
required for using gyro sensor.

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