dt-bindings: usb: dwc2: add description for px30

This patch adds the compatible of dwc2 for PX30 SoCs.

Signed-off-by: Liang Chen <cl@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>