cpuidle: sysfs: Export target residency information

From user space, there is no way to know the target residency for each idle
state. If we want to write tools to measure the accuracy of the idle state
selection from the governor, we need this info.

As the exit latency is exported through sysfs, exporting the target residency
in the same place makes sense.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>