cpuidle: record state entry rejection statistics

CPUs may fail to enter the chosen idle state if there was a
pending interrupt, causing the cpuidle driver to return an error
value.

Record that and export it via sysfs along with the other idle state
statistics.

This could prove useful in understanding behavior of the governor
and the system during usecases that involve multiple CPUs.

Signed-off-by: Lina Iyer <ilina@codeaurora.org>
[ rjw: Changelog and documentation edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>