iio: core : events ABI for specifying period

The iio sysfs ABI defines a way to specify period for roc and thresholds.
What:		/sys/.../events/in_accel_x_thresh_rising_period
What:		/sys/.../events/in_accel_x_thresh_falling_period
what:		/sys/.../events/in_accel_x_roc_rising_period
What:		/sys/.../events/in_accel_x_roc_falling_period

But there is no way to add period with the current event info enum.

Added IIO_EV_INFO_PERIOD and corresponding string.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>