crypto: qat - add ring buffer idle telemetry counter for GEN6

Add a new performance counter that measures the average ring buffer idle
duration.

This metric is now included in the telemetry counters exposed via
debugfs for QAT GEN6 devices.

Update the documentation to reflect the new idle duration counter

Co-developed-by: George Abraham P <george.abraham.p@intel.com>
Signed-off-by: George Abraham P <george.abraham.p@intel.com>
Signed-off-by: Vijay Sundar Selvamani <vijay.sundar.selvamani@intel.com>
Signed-off-by: Suman Kumar Chakraborty <suman.kumar.chakraborty@intel.com>
Reviewed-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>