dm: core: Handle global_data moving in SPL

When CONFIG_SPL_STACK_R is enabled, and spl_init() is called before
board_init_r(), spl_relocate_stack_gd() will move global_data to a new
place in memory. This affects driver model since it uses a list for the
uclasses. Unless this is updated the list will become invalid. When
looking for a non-existent uclass, such as when adding a new one, the loop
in uclass_find() may continue forever, thus causing a hang.

Add a function to correct this rather obscure bug.

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