dm: core: Support allocating driver-private data for DMA

Some driver want to put DMA buffers in their private data. Add a flag
to tell driver model to align driver-private data to a cache boundary so
that DMA will work correctly in this case.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Marek Vasut <marex@denx.de>