It works when CONFIG_USING_KERNEL_DTB is enabled, because we can
set either 'u-boot,dm-spl' or 'u-boot,dm-pre-reloc' for the node
that we can to leave in the u-boot.dtb.
Change-Id: Id3cec0c64ee55ca9bc1e5871e7ad5a2ebb4b1b6a
Signed-off-by: Joseph Chen <chenjh@rock-chips.com>