Documentation: qat: update sysfs-driver-qat for GEN6 devices

Reorganize and expand documentation for service configurations. This
reworks the `cfg_services` section by removing explicit service
combinations (e.g., asym;sym, sym;asym) and clarifying that multiple
services can be configured in any order.

Update the documentation to reflect that the attribute previously limited
to qat_4xxx devices is also applicable to qat_6xxx devices.

Signed-off-by: Suman Kumar Chakraborty <suman.kumar.chakraborty@intel.com>
Reviewed-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Reviewed-by: Fiona Trahe <fiona.trahe@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>