phy: rockchip-usb: add usb-uart setup for rk3188

The rk3188 also supports bringing the uart2 out through
the usb dm+dp pins, so add the necessary setup for it.

rk3066 does not seem to support usb-uart functionality and this
particular phy was only used on older Rockchip socs, so this leaves
room for a bit of cleanup as well, as there most likely won't be new
additions in the driver.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>