iio: adxl345: Add support for the ADXL375

The ADXL375 is fully register map compatible to the ADXL345 (including the
device ID register returning the same value ...).

The only difference is the resolution of the acceleration sensor. The
ADXL375 can measure up to +-200g of acceleration.

Datasheet:
http://www.analog.com/media/en/technical-documentation/data-sheets/ADXL375.PDF

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mircea Caprioru <mircea.caprioru@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>