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Add a timestamping mechanism for buffer that provides accurate
event timestamps when using watermark. This mechanism estimates
device internal clock by comparing FIFO interrupts delta time and
device elapsed time computed by parsing FIFO data.

Take interrupt timestamp in hard irq handler and add IIO device
specific timestamp structures in device private allocation.
Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>