ARM: debug: add low level debug uart for rv1108

RK1108 UARTs are Synopsis DesignWare 8250 compatible.
Only with different register addresses.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Tested-by: Jacob Chen <jacob2.chen@rock-chips.com>
[rk1108->rv1108 rename]
Signed-off-by: Heiko Stuebner <heiko@sntech.de>