arm64: dts: rockchip: rk3588s-rock5a: add USB3 host

Enable USB3 host controller for the Radxa ROCK 5 Model A. This adds
USB3 for the lower USB3 port (the one closer to the PCB).

The upper USB3 port uses the RK3588 USB TypeC host controller, which
uses a different PHY that is not yet supported upstream.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>