cpuidle: Use s64 as exit_latency_ns and target_residency_ns data type

Subsequent changes will cause the exit_latency_ns and target_residency_ns
fields in struct cpuidle_state to be used in computations in which data
type conversions to u64 may turn a negative number close to zero into
a verly large positive number leading to incorrect results.

In preparation for that, change the data type of the fields mentioned
above to s64, but ensure that they will not be negative themselves.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>