iio: hrtimer: Move sampling_frequency to trigger dir

sampling_frequency is more an attribute of the IIO trigger
so we move the configuration from
/config/triggers/hrtimer-instanceX/sampling_frequency/
to coresponding sysfs file:
/sys/bus/iio/devices/triggerY/sampling_frequency.

While at it add few const to various structures as per review
on linux-iio list.

Change-Id: Iecf9db7480a1f08efea9c92c86370f16859aaa1e
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
2 files changed