rockchip: imply instead of selecting SPL_SYS_MALLOC_SIMPLE

We shouldn't force which allocator the SPL uses, since there's no
platform requirement for one over the other: in fact, we currently allow
selection of the TPL allocator but not the SPL one!

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>