crypto: qat - expose pm_idle_enabled through sysfs

Expose 'pm_idle_enabled' sysfs attribute. This attribute controls how
idle conditions are handled. If it is set to 1 (idle support enabled)
when the device detects an idle condition, the driver will transition
the device to the 'MIN' power configuration.

In order to set the value of this attribute for a device, the device
must be in the 'down' state.

This only applies to qat_4xxx generation.

Signed-off-by: Lucas Segarra Fernandez <lucas.segarra.fernandez@intel.com>
Reviewed-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>