dm: of-platadata: Add option for device instantiation

dm: of-platadata: Add option for device instantiation

Add Kconfig options to support build-time device instantiation. When
fully implemented, this will allow dtoc to create U-Boot devices (i.e.
struct udevice records) at build time, thus reducing code space in
SPL.

For now this defaults to off, but will be enabled when the rest of
the implementation is in place.

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