crypto: qat - add num_rps sysfs attribute

Add the attribute `num_rps` to the `qat` attribute group. This returns
the number of ring pairs that a single device has. This allows to know
the maximum value that can be set to the attribute `rp2svc`.

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>