iio: imu: adis16480: Calculate the sampling frequency in a generic way

The adis1648x devices have an internal clock of 2.46 kSPS. The sampling
frequency is calculated by applying a decimation rate which can take the
maximum value of 2047.

Although all adis1648x devices are similar in this regard, devices that
will use this feature will be added in the future.

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