iio: trigger: Introduce IIO hrtimer based trigger

This patch registers a new IIO software trigger interrupt source
based on high resolution timers.

Notice that if configfs is enabled we create sampling_frequency
attribute allowing users to change hrtimer period (1/sampling_frequency).

The IIO hrtimer trigger has a long history, this patch is based on
an older version from Marten and Lars-Peter.

Signed-off-by: Marten Svanfeldt <marten@intuitiveaerial.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>