dt-bindings: rcar-gen3-phy-usb2: Document dr_mode

Document the optional dr_mode property

Signed-off-by: Chris Brandt <chris.brandt@renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>