rockchip: rk3188: add support for usb-uart functionality

rockchip: rk3188: add support for usb-uart functionality

Rockchip socs can route the debug uart pins through the d+ and d- pins
of one specific usbphy per soc. Add a config option and implement the
setting on the rk3188.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
[Fixed up to mark grf as maybe unused:]
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>