dt-bindings: usb: ohci: Add clocks description for R-Car Gen3

This patch adds detailed information of an optional property "clocks"
description for R-Car Gen3.
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/usb-ohci.txt

@@ -12,7 +12,11 @@ Optional properties:
 - no-big-frame-no : boolean, set if frame_no lives in bits [15:0] of HCCA
 - remote-wakeup-connected: remote wakeup is wired on the platform
 - num-ports : u32, to override the detected port count
-- clocks : a list of phandle + clock specifier pairs
+- clocks : a list of phandle + clock specifier pairs. In case of Renesas
+	   R-Car Gen3 SoCs:
+	   - if a host only channel: first clock should be host.
+	   - if a USB DRD channel: first clock should be host and second one
+				   should be peripheral.
 - phys : see usb-hcd.txt in the current directory
 - resets : a list of phandle + reset specifier pairs