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>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>