iio:adxl372: Add filter bandwidth support

This patch adds the option for the user to select the filter bandwidth. The
user can also read the available bandwidths which are always adjusted to be
at most half of the sampling frequency. Furthermore, the currently selected
bandwidth can be read via the read_raw function, while the write_raw sets a
new bandwidth value.

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