dt-bindings: phy: phy-rockchip-inno-usb2: add rk3568 documentation

The rk3568 usb2phy node is a standalone node with a single muxed
interrupt.
Add documentation for it to phy-rockchip-inno-usb2.

Signed-off-by: Peter Geis <pgwipeout@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211215210252.120923-3-pgwipeout@gmail.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>