ram: rockchip: Fix dependency of RAM_ROCKCHIP_DEBUG

ram: rockchip: Fix dependency of RAM_ROCKCHIP_DEBUG

The RAM_ROCKCHIP_DEBUG can be used only if DEBUG_UART is
available.
The next commit introduces changes in definition of debug
uart functions, so that DEBUG_UART is required to be defined
in order to initialize uart and use print functions.

Signed-off-by: Lukasz Czechowski <lukasz.czechowski@thaumatec.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>