dm: core: Allow storing priv/plat data separately

dm: core: Allow storing priv/plat data separately

At present the device priv/data data allocated by dtoc is stored in the
data section along with other variables. On some platforms it is better
to allocate space for it separately, e.g. if SPL is running from read-only
memory.

Create a new space with the same size as that allocated by dtoc, ready for
use.

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