powerpc: of_parse_dma_window should take a __be32 *dma_window

We pass dma_window to of_parse_dma_window as a void * and then
run through hoops to cast it back to a u32 array. In the process
we lose endian annotation.

Simplify it by just passing a __be32 * down.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>