Handling TPL and SPL in the Makefile for mach-rockchip was based on
nested if checks and/or if-else-if paths.  This can be simplified and
made more readable by using $(SPL_TPL_) and by introducing
intermediate variables for the aggregation of SPL and TPL features.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>