ram: Mark ram-uclass depend on TPL_DM or SPL_DM

ram-uclass is building irrespective of whether TPL_DM
or SPL_DM is enabled. So control the ram uclass build
based on TPL/SPL_DM.

Signed-off-by: Jagan Teki <jagan@edgeble.ai>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>