dm: core: Allow marking driver model as dead

dm: core: Allow marking driver model as dead

On x86 devices we use CAR (Cache-As-RAM) to hold the malloc() region in
SPL, since SDRAM is not set up yet. This means that driver model stores
its tables in this region.

When preparing to jump from SPL to U-Boot proper, we must disable CAR, so
that the CPU can uses the caches normally. This means that driver model
tables become inaccessible. From there until we jump to U-Boot proper, we
must avoid using driver model.

This is only a problem on boards which operate this way, for example
chromebook_link64

Add a flag to indicate that driver model is dead and should not be used.
It can be used in SPL to avoid hanging the machine.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>