rockchip: rk3188: Cleanup some SPL/TPL rename leftovers

In the beginning, we did SPL -> TPL -> U-Boot, but after clarification
of the real ordering swapped SPL and TPL.
It seems some renames were forgotten and may confuse future readers, so
also swap these to reflect the actual ordering.

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