crypto: qat - add rp2svc sysfs attribute

Add the attribute `rp2svc` to the `qat` attribute group. This provides a
way for a user to query a specific ring pair for the type of service
that is currently configured for.

When read, the service will be returned for the defined ring pair.
When written to this value will be stored as the ring pair to return
the service of.

Signed-off-by: Ciunas Bennett <ciunas.bennett@intel.com>
Reviewed-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Reviewed-by: Damian Muszynski <damian.muszynski@intel.com>
Reviewed-by: Tero Kristo <tero.kristo@linux.intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>