rockchip: handle peripheral as well as otg dr_mode

The OTG port is identified by inspecting the "dr_mode" property which is
expected to be "otg" for this port.  But it will work just as well as a
device controller when dr_mode is set to "peripheral", which may be
required if the mode detection pin is not set up correctly and the
device controller needs to be programmed to override this.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>