doc: usb: ci-hdrc-usb2: Add pinctrl properties definition

Some hardware implementations require to configure pins differently
according to the USB role (host/device), this can be an update of the
pins routing or a simple GPIO value change.

This patch introduces new optional "host" and "device" pinctrls.
If these pinctrls are defined by the device, they are respectively
selected on host/device role start.
Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Peter Chen <peter.chen@nxp.com>

Documentation/devicetree/bindings/usb/ci-hdrc-usb2.txt

@@ -80,6 +80,8 @@ Optional properties:
   controller. It's expected that a mux state of 0 indicates device mode and a
   mux state of 1 indicates host mode.
 - mux-control-names: Shall be "usb_switch" if mux-controls is specified.
+- pinctrl-names: Names for optional pin modes in "default", "host", "device"
+- pinctrl-n: alternate pin modes
 
 i.mx specific properties
 - fsl,usbmisc: phandler of non-core register device, with one