Add a function for generic parsing of dma-window properties (ie,
ibm,dma-window and ibm,my-dma-window) of pci and virtual device nodes.

This function will also be used by cell.
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>