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>