dm: core: Fix Handling of global_data moving in SPL

commit 2f11cd91 ("dm: core: Handle global_data moving in SPL")
handles relocation of GD in SPL if spl_init() is called before
board_init_r(). So, uclass_root.next need not be initialized always
and accessing uclass_root.next->prev gives an abort. Update the
uclass_root only if it is available.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>