dt-bindings: soc: rockchip: add rk3568-usb2phy-grf

Add the documentation for the rk3568-usb2phy-grf node, which is separate
from the usb2phy node on this chip.

Signed-off-by: Peter Geis <pgwipeout@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211215210252.120923-2-pgwipeout@gmail.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>