drivers: spl: consistently use the $(SPL_TPL_) macro

To simplify drivers/Makefile a bit when using TPL/SPL, we consistently
use the $(SPL_TPL_) macro to test for drivers that have separate
configuration symbols for the full U-boot, SPL and TPL stages.
Instead of explicitly repeating them in two separate if-guarded
sections of the Makefile, we can now simply list these options once.

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