rockchip: Enable pre console for rk3399

Enable pre console buffer for rk3399 platform.

This would help to capture the console messages prior to
the console being initialised. Enabling this would help
to capture all the console messages on video output source
like HDMI. So we can find the full console messages of
U-Boot proper on HDMI display when enabled it for RK3399
platform boards.

Buffer address used for pre console is 0x0f200000 which is
ram base plus 240MiB. right now the Allwinner SoC is using
similar computation.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>