s390/cpuinfo: show maximum thread id

Expose the maximum thread id with /proc/cpuinfo.
With the new line the output looks like this:

vendor_id       : IBM/S390
bogomips per cpu: 20325.00
max thread id   : 1

With this new interface it is possible to always tell the correct
number of cpu threads potentially being used by the kernel.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>