This adds support for PEAK-System PCAN-USB X6 USB to CAN interface.

The CAN FD adapter PCAN-USB X6 allows the connection of up to 6 CAN FD
or CAN networks to a computer via USB. The interface is installed in an
aluminum profile casing and is shipped in versions with D-Sub connectors
or M12 circular connectors.

The PCAN-USB X6 registers in the USB sub-system as if 3x PCAN-USB-Pro FD
adapters were plugged. So, this patch:

- updates the PEAK_USB entry of the corresponding Kconfig file
- defines and adds the device id. of the PCAN-USB X6 (0x0014) into the
  table of supported device ids
- defines and adds the new software structure implementing the PCAN-USB X6,
  which is obviously a clone of the software structure implementing the
  PCAN-USB Pro FD.
Signed-off-by: Stephane Grosjean <s.grosjean@peak-system.com>
Tested-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>