spl: Kconfig: split SYS_MALLOC_SIMPLE for TPL and SPL

As include/malloc.h already checks for SYS_MALLOC_SIMPLE using the
CONFIG_IS_ENABLED macro, we need to move to having separate entries
as we switch to fully separate configuration for SPL and TPL.

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