usb: xhci: plat: Add USB 3.0 phy support

For Realtek SoC, the usb xhci uses different driver for u2phy and u3phy.
Therefore, add a hook to retrieve the USB 3.0 PHY to XHCI plat.

Signed-off-by: Stanley Chang <stanley_chang@realtek.com>
Link: https://lore.kernel.org/r/20230407060731.20537-1-stanley_chang@realtek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>