s390/zcrypt: Number of supported ap domains is not retrievable.

Upcoming versions of secure key management facilities (CCA and
EP11) require information about the maximum number of supported
ap domains in order to service TKE requests properly. With IBM
z13 the number of available domains (so far 16) has increased up
to 85. This number varies depending on machine types and models.
Therefore the new sysfs attribute 'ap_max_domain_id' provides
this limit of supported ap domains.  Upcoming releases for CCA
and EP11 will use this new information. Without this problem fix
it is not possible to retrieve reliable information about the
maximum number of supported ap domains. Thus, customers are not
able to perform key management for CCA and EP11 coprocessor
adapters.

Signed-off-by: Ingo Tuchscherer <ingo.tuchscherer@de.ibm.com>
Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>