s390/pci: speed up __iowrite64_copy by using pci store block insn

Benefit from pci store block instruction by writing up to 128 bytes
with a single instruction to MMIO space. Depending on the workload
this can result in a huge performance increase due to the reduced
number of instructions. The ordering guarantees of single stores
vs. one store block are identical.

Signed-off-by: Jan Glauber <jang@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>