dt-bindings: phy: realtek: Add Realtek DHC RTD SoC USB 3.0 PHY

Document the USB PHY bindings for Realtek SoCs.
Realtek DHC (digital home center) RTD SoCs support DWC3 XHCI USB
controller and using USB 3.0 PHY transceiver.

Signed-off-by: Stanley Chang <stanley_chang@realtek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230725033318.8361-5-stanley_chang@realtek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>