iio: accel: Add support for Freescale MMA7660FC

Minimal implementation of an IIO driver for the Freescale
MMA7660FC 3-axis accelerometer. Datasheet:
http://www.nxp.com/files/sensors/doc/data_sheet/MMA7660FC.pdf



Includes:
- ACPI support;
- read_raw for x,y,z axes;
- reading and setting the scale (range) parameter.
- power management

Signed-off-by: Constantin Musca <constantin.musca@intel.com>
Reviewed-by: Martin Klepplinger <martink@posteo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>