spl: fix TPL_SYS_MALLOC_F description

This config option enables the malloc() pool in TPL not the SPL.  Fix
the description to accurately reflect this.

Fixes: fd8497da (spl: Create proper symbols for enabling the malloc() pool)
Signed-off-by: John Keeping <jkeeping@inmusicbrands.com>
Reviewed-by: Simon Glass <sjg@chromium.org>