iio: accel: Add support for Sensortek STK8312

Minimal implementation of an IIO driver for the Sensortek
STK8312 3-axis accelerometer. Datasheet:
http://www.syi-group.com/uploadpic/data/201361817562681623.pdf



Includes:
- ACPI support;
- read_raw for x,y,z axes;
- reading and setting the scale (range) parameter.
- power management

Signed-off-by: Tiberiu Breana <tiberiu.a.breana@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>