[POWERPC] Add of_translate_dma_address

This adds a variant of of_translate_address that uses the dma-ranges
property instead of "ranges", it's to be used by PCI code in parsing
the dma-ranges property.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>