configs: rock-pi-4: Enable LPDDR4 support
Due to foot-print issues, we have LPDDR4 code can be
marked as CONFIG_RAM_RK3399_LPDDR4.
So, enable it for Rock-PI-4 board.
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <Kever.yang@rock-chips.com>