powerpc: Remove map_/unmap_single() from dma_mapping_ops

Now that all of the remaining dma_mapping_ops have had their
map_/unmap_single functions updated to become map/unmap_page
functions, there is no need to have the map_/unmap_single function
pointers in the dma_mapping_ops.

So, this removes them and also removes the code that does the checking
for which set of functions to use.

Signed-off-by: Mark Nelson <markn@au1.ibm.com>
Acked-by: Becky Bruce <becky.bruce@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>