iio:adxl372: Add FIFO and interrupts support

This patch adds support for the adxl372 FIFO. In order to accomplish this,
triggered buffers were used.

The number of FIFO samples which trigger the watermark interrupt can be
configured by using the buffer watermark. The FIFO format is determined by
configuring the scan elements for each axis. The FIFO data is pushed to the
IIO device's buffer.

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