iommu: Add DMA window parser, of_get_dma_window()

This code was based on:
    "arch/microblaze/kernel/prom_parse.c"
    "arch/powerpc/kernel/prom_parse.c"

Can replace "of_parse_dma_window()" in the above. This supports
different formats flexibly. "prefix" can be configured if any. "busno"
and "index" are optionally specified. Set NULL and 0 if not used.

Signed-off-by: Hiroshi DOYU <hdoyu@nvidia.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>