powerpc/iommu: Only build sPAPR access functions on pSeries

 and PowerNV

A build failure with CONFIG_HAVE_PCI=y set without PSERIES or POWERNV
set was caught by the random configuration checker.  Guard the sPAPR
specific IOMMU functions on CONFIG_PPC_PSERIES || CONFIG_PPC_POWERNV.

Signed-off-by: Timothy Pearson <tpearson@raptorengineering.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/2015925968.3546872.1685990936823.JavaMail.zimbra@raptorengineeringinc.com