cpuidle: driver: Update microsecond values of state parameters as needed

If the cpuidle driver provides the target residency and exit latency in
nanoseconds, the corresponding values in microseconds need to be set to
reflect the provided numbers in order for the sysfs interface to show
them correctly, so make __cpuidle_driver_init() do that.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>