iio: accel: Add buffer mode for Sensortek STK8312

Added triggered buffer mode support for the STK8312 accelerometer.

Additional changes:
- set_mode now sets operation mode directly, no longer masking
  the register's previous value
- read_accel now returns raw acceleration data instead of the
  sign_extend32 value
- read_raw will now enable/disable the sensor with each reading

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