usb: dwc3: Support Synopsys USB 3.1 IP

This patch allows the dwc3 driver to run on the new Synopsys USB 3.1
IP core, albeit in USB 3.0 mode only.

The Synopsys USB 3.1 IP (DWC_usb31) retains mostly the same register
interface and programming model as the existing USB 3.0 controller IP
(DWC_usb3). However the GSNPSID and version numbers are different.

Add checking for the new ID to pass driver probe.

Also, since the DWC_usb31 version number is lower in value than the
full GSNPSID of the DWC_usb3 IP, we set the high bit to identify
DWC_usb31 and to ensure the values are higher.

Finally, add a documentation note about the revision numbering scheme.
Any future revision checks (for STARS, workarounds, and new features)
should take into consideration how it applies to both the 3.1/3.0 IP.

Cc: <stable@vger.kernel.org> # v3.18+
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>