s390: Implement __iowrite32_copy()

It is trivial to implement an inline to do this, so provide it in the s390
headers. Like the 64 bit version it should just invoke zpci_memcpy_toio()
with the correct size.

Link: https://lore.kernel.org/r/2-v3-1893cd8b9369+1925-mlx5_arm_wc_jgg@nvidia.com


Acked-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>