Enable Super speed plus in configfs to support USB3.1 Gen2.
This ensures that when a USB gadget is plugged in, it is
enumerated as Gen 2 and connected at 10 Gbps if the host and
cable are capable of it.

Many in-tree gadget functions (fs, midi, acm, ncm, mass_storage,
etc.) already have SuperSpeed Plus support.

Tested: plugged gadget into Linux host and saw:
[284907.385986] usb 8-2: new SuperSpeedPlus Gen 2 USB device number 3 using xhci_hcd
Tested-by: Lorenzo Colitti <lorenzo@google.com>
Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: taehyun.cho <taehyun.cho@samsung.com>
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Link: https://lore.kernel.org/r/20210106154625.2801030-1-lorenzo@google.com
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>