s390/pci: unify pci_iomap symbol exports

Since commit 8cfc99b5

 ("s390: add pci_iomap_range") we use
EXPORT_SYMBOL for pci_iomap but EXPORT_SYMBOL_GPL for pci_iounmap.
Change the related functions to use EXPORT_SYMBOL like the asm-generic
variants do.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>