usb: dwc3: Add driver for Xilinx platforms

Add a new driver for supporting Xilinx platforms. This driver is used
for some sequence of operations required for Xilinx USB controllers.
This driver is also used to choose between PIPE clock coming from SerDes
and the Suspend Clock. Before the controller is out of reset, the clock
selection should be changed to PIPE clock in order to make the USB
controller work. There is a register added in Xilinx USB controller
register space for the same.

Signed-off-by: Manish Narani <manish.narani@xilinx.com>
Link: https://lore.kernel.org/r/1615963949-75320-3-git-send-email-manish.narani@xilinx.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>