s390: increase CONFIG_NR_CPUS limit

In current models, maximum number of active cores is 101.

[heiko.carstens@de.ibm.com]: Xose's patch increased the maximum possible
value of CONFIG_NR_CPUS to 101. I changed this to 256 instead.

Signed-off-by: Xose Vazquez Perez <xose.vazquez@gmail.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>