ARM: rockchip: clarify usability of DEBUG_RK3X_UART debug_ll options

The debug uart settings from the DEBUG_RK3X_UART options are usable on
all Rockchip SoCs from the rk30xx and rk31xx series but not on the
new rk3288 SoCs. Thus clarify their use to prevent confusion.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>