dt-bindings: usb: renesas_usbhs: add clock-names property

R-Car Gen3 needs to enable clocks of both host and peripheral.
Otherwise, other side device cannot work correctly. So, this patch
adds a property of clock-names for R-Car Gen3 as an optional.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

Documentation/devicetree/bindings/usb/renesas_usbhs.txt

@@ -25,7 +25,11 @@ Required properties:
 
   - reg: Base address and length of the register for the USBHS
   - interrupts: Interrupt specifier for the USBHS
-  - clocks: A list of phandle + clock specifier pairs
+  - clocks: A list of phandle + clock specifier pairs.
+	    - In case of "renesas,rcar-gen3-usbhs", two clocks are required.
+	      First clock should be peripheral and second one should be host.
+	    - In case of except above, one clock is required. First clock
+	      should be peripheral.
 
 Optional properties:
   - renesas,buswait: Integer to use BUSWAIT register