usb: otg: support for multiple transceivers by a single controller

Add a linked list for keeping multiple PHY instances with different
types so that we can have separate USB2 and USB3 PHYs on one single
board. _get_phy_ has been changed so that the controller gets
the transceiver by type. _remove_phy_ has been added to let the phy
be removed from the phy list.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>