iio: accel: Add support for Sensortek STK8BA50

Minimal implementation of an IIO driver for the Sensortek
STK8BA50 3-axis accelerometer. Datasheet:
http://szgsensor.com/uploads/soft/141229/STK8BA50%D2%E5%BC%CE.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>