At present when driver model needs to change a device it simply updates
the struct udevice structure. But with of-platdata-inst most of the fields
are not modified at runtime. In fact, typically only the flags need to
change.

For systems running SPL from read-only memory it is convenient to separate
out the runtime information, so that the devices don't need to be copied
before being used.

Create a new udevice_rt table, similar to the existing driver_rt. For now
it just holds the flags, although they are not used in this patch.

Add a new Kconfig for the driver_rt data, since this is not needed when
of-platdata-inst is used.

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