Revert "Increase default of CONFIG_SYS_MALLOC_F_LEN for SPL_OF_CONTROL"

This reverts commit 90c08d9e.

I took a closer look at this after the commit was applied, and found
CONFIG_SYS_MALLOC_F_LEN=0x2000 was too much.  8KB memory for SPL is
actually too big for some boards.  Perhaps 0x800 is enough, but the
situation varies board by board.

Let's postpone our decision until we come up with a better idea.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>