iio: accel: Add driver for the Analog Devices ADXL345 3-axis accelerometer

Add basic IIO support for the Analog Devices ADXL345 3-axis accelerometer.
The datasheet can be found here:
http://www.analog.com/media/en/technical-documentation/data-sheets/ADXL345.pdf



Signed-off-by: Eva Rachel Retuya <eraretuya@gmail.com>
Cc: Michael Hennerich <michael.hennerich@analog.com>
Cc: Daniel Baluta <daniel.baluta@gmail.com>
Cc: Alison Schofield <amsfield22@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>