dm: core: Ensure DMA regions start up with the cache clean

There is a strange interaction with drivers which use DMA if the cache
starts off in a dirty state. Buffer space which the driver reads (but has
not previously written) can contain zero bytes from alloc_priv(). This can
cause corruption of the memory used by DMA for incoming data.

Fix this and add a comment to explain the problem.

This allows the dwc2 driver to work correctly with driver model, for
example.

Signed-off-by: Simon Glass <sjg@chromium.org>