dm: core: Allow the uclass list to move

At present the uclass list head is in global_data. This is convenient
but with the new of-platdata we need the list head to be declared by
the generated code.

Change this over to be a pointer. Provide a 'static' version in
global_data to retain the current behaviour.

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