iio: accel: adxl380: Add support for 1 kHz sampling frequency

iio: accel: adxl380: Add support for 1 kHz sampling frequency

In sensor variants (such as ADXL380 and ADXL382) that support low-power
mode, the SAR signal path allows sampling acceleration data at lower rates;
more specifically, when the sensor operates in VLP mode, the sampling
frequency is 1 kHz.

To add support for the 1kHz sampling frequency value, modify the operating
mode selection logic to take into account the sampling frequency, and
configure the decimation filters only when applicable (i.e. when using a
sampling frequency that relies on the DSM signal path); in addition,
constrain the available sampling frequency values based on whether the
sensor is operating in low-power mode.

Signed-off-by: Francesco Lavra <flavra@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>