arm: socfpga: config: Add USB support example

Add example configuration stub for the DWC2 USB controller.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>
Cc: Vince Bridgers <vbridger@altera.com>
Cc: Pavel Machek <pavel@denx.de>