s390x/cpumodel: document S390FeatDef.bit not applicable

The 'bit' field of the 'S390FeatDef' structure is not applicable to all
its instances. Currently this field is not applicable, and remains
unused, iff the feature is of type S390_FEAT_TYPE_MISC. Having the value 0
specified for multiple such feature definitions was a little confusing,
as it's a perfectly legit bit value, and as the value of the bit
field is usually ought to be unique for each feature of a given
feature type.

Let us introduce a specialized macro for defining features of type
S390_FEAT_TYPE_MISC so, that one does not have to specify neither bit nor
type (as the latter is implied).

Signed-off-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Message-Id: <20180221165628.78946-1-pasic@linux.vnet.ibm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>