Currently usb3 port in fact includes two sub-ports, but it is not
flexible for some cases, such as following one:
    usb3 port0 includes u2port0 and u3port0;
    usb2 port0 includes u2port1;
If wants to support only HS, we can use u2port0 or u2port1, when
select u2port0, u3port0 is not needed;
If wants to support SS, we can compound u2port0 and u3port0,
or u2port1 and u3port0, if select latter one, u2port0 is not needed.

So it's more flexible to split usb3 port into two ones and also try
best to save power by disabling unnecessary ports.
Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>