powerpc/iommu: Rename iommu_[un]map_sg functions

The IOMMU-API gained support for a new iommu_map_sg
function. This causes compile failures on powerpc because
the function name is already globally used there.
This patch renames adds a ppc_ prefix to these functions to
solve the compile problem.

Signed-off-by: Joerg Roedel <jroedel@suse.de>