usb: dwc3-generic: add a new host driver that uses the dwc3 core

usb: dwc3-generic: add a new host driver that uses the dwc3 core

Currently the host driver used by dwc3-generic is "xhci-dwc3". This is
a functional driver but it doesn't use the dwc3 core and, in particular,
it lacks some bits that may be important.
For example on the k2 platforms, it is important that the phy are properly
suspended when the USB is not used anymore. The dwc3 core also has a
partial support for quirks.
The new driver can be used as a drop-in replacement for "xhci-dwc3".

In terms of implementation, it may seem strange that 2 private structures
dwc3_generic_host_priv and dwc3_generic_priv) are used. The reason for this
is simply that the xhci layer expects a struct xhci_ctrl at the beginning
of the private data and it seemed wasteful to include it also for the
peripheral case.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>