arm: add debug uarts for rockchip rk29xx and rk3xxx series

Uarts on all recent Rockchip SoCs are Synopsis DesignWare 8250 types.
Only their addresses vary very much.

This patch adds the necessary definitions to use any of the uart ports
for early debug purposes.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>