iio: accel: Add trigger support for STK8BA50

Add data-ready interrupts and trigger support for STK8BA50.

Additional changes:
- read_accel now returns raw acceleration data instead of the
  sign_extend32 value
- read_raw will now enable/disable the sensor with each reading

Change-Id: I9c2d7be4256b2dcc5546e4432308ea54f8004333
Signed-off-by: Tiberiu Breana <tiberiu.a.breana@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>