spl: use TPL_SYS_MALLOC_F_LEN for TPL

The (upstream) changes to break up SYS_MALLOC_F_LEN for the full
U-Boot and the SPL stage, break TPL (if simple malloc is enabled in
TPL).

This adds support for a TPL-variant of SYS_MALLOC_F_LEN:
- adds TPL_SYS_MALLOC_F_LEN
- rewrites a test for CONFIG_SPL_SYS_MALLOC_F_LEN to access
  CONFIG_VAL(SYS_MALLOC_F_LEN)

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