s390/Kconfig: default PCI_NR_FUNCTIONS to 512

With the addition of more complete SR-IOV support, we are recommending
to raise this limit for distributions to 512, as the previous default of
128 can easily be hit with just the VFs of a single PCI physical
function.
With at least one distribution now shipping with this, supporting
only one fourth as many PCI functions on a default upstream build may
lead to confusion and reduced testing of the higher limit so increase
the default to 512.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>