dt-bindings: usb: mtk-xhci: add optional property to disable usb2 ports

Add support to disable specific usb2 host ports, it's useful when
a usb2 port is disabled on some platforms, but enabled on others for
the same SoC, another case is that the different package may support
different number of ports.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/1629189389-18779-1-git-send-email-chunfeng.yun@mediatek.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>