usb: dwc3: add a flag to check if it is fpga board

Some chip vendor is on pre-silicon phase, which needs to use the simulation
board. It should have the same product and vendor id with the true soc, but
might have some minor different configurations.

Below thread discussion proposes to find a method to distinguish between
simulation board and soc.

http://marc.info/?l=linux-usb&m=141194772206369&w=2



In Andvanced Configuration of coreConsultant, there is the parameter of
DWC_USB_EN_FPGA. This bit has the function we need. And it would response as 7
bit of GHWPARAMS6 register. So it's able to check this functional bit to confirm
if works on FPGA board.

Reported-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>