rockchip: Print a message when returning to the bootrom

At present if the return to bootrom fails (e.g. because you are not using
the Rockchip's bootrom's pointer table in MMC) then the board prints
SPL message and hangs. Print a message first if we can, to help in
understanding what happened when it hangs.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Heiko Stuebner <heiko@sntech.de>