rockchip: rv1108: disable CONFIG_RAM before we have driver

The rv1108 do not have DRAM driver now, so disable it first,
or else it will get conflict with the sdram common code.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>