rockchip: serial: Adapt rockchip of-platdata driver for rk3188

Add necessary structs to have the driver also work for the serial
on the rk3188.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Kever Yang <kever.yang@rock-chips.com>