powerpc: setup default archdata for {of_}platform via bus_register_notifier

Since a number of powerpc chips are SoCs we end up having dma-able
devices that are registered as platform or of_platform devices.  We need
to hook the archdata to setup proper dma_ops for these devices.

Rather than having to add a bus_notify to each platform we add a default
one at the highest priority (called first) to set the default dma_ops for
of_platform and platform devices to dma_direct_ops.  This allows platform
code to override the ops by providing their own notifier call back.

In the future to enable >4G DMA support on ppc32 we can hook swiotlb ops.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>