dm: core: Use separate priv/plat data region

Make use of the new priv/plat data region if enabled. This is implemented
as a simple offset from the position set up by dtoc to the new position.

So long as all access goes through dm_priv_to_rw() this is safe.

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