dt-bindings: usb: add rk3568 compatible to rockchip, dwc3

Add the compatible for the Rockchip RK3568 variant.

Tested-by: Michael Riesch <michael.riesch@wolfvision.net>
Signed-off-by: Michael Riesch <michael.riesch@wolfvision.net>
Link: https://lore.kernel.org/r/20220228135700.1089526-4-pgwipeout@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>