powerpc/dma: remove dma_nommu_mmap_coherent

The coherent cache version of this function already is functionally
identicall to the default version, and by defining the
arch_dma_coherent_to_pfn hook the same is ture for the noncoherent
version as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Christian Zigotzky <chzigotzky@xenosoft.de>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>