iio: adxl372: New driver for Analog Devices ADXL372 Accelerometer

This patch adds basic support for Analog Devices ADXL372 SPI-Bus
Three-Axis Digital Accelerometer.

The device is probed and configured the with some initial default
values. With this basic driver, it is possible to read raw acceleration
data.

Datasheet:
http://www.analog.com/media/en/technical-documentation/data-sheets/ADXL372.pdf

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>