USB: at91: Add USB gadget driver selection for at91sam9g45 series

Add gadget USB drivers for at91sam9g45 series. Those SOC include
high speed USB interfaces.
The gadget driver is the already available atmel_usba_udc.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>