iio: accel: fxls8962af: add hw buffered sampling

When buffered sampling is enabled, the accelerometer will dump data into
the internal fifo and interrupt at watermark. Then the driver flushes
all data to the iio buffer.
As the accelerometer doesn't have internal timestamps, they are
approximated between the current and last interrupt.

Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>