Staging: IIO: lis3l02dq accelerometer core support

A later patch in the series will add data ready triggering
and ring buffer support.

This core patch provides an event interface and sysfs
based reading of values.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>