Convert CONFIG_USB_GADGET_AT91 to Kconfig

This converts the following to Kconfig:
   CONFIG_USB_GADGET_AT91

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig
index e8da73c..e120efe 100644
--- a/drivers/usb/gadget/Kconfig
+++ b/drivers/usb/gadget/Kconfig
@@ -82,6 +82,10 @@
 	help
 	  Enable the Broadcom UDC OTG physical device interface.
 
+config USB_GADGET_AT91
+	bool "Atmel AT91 USB Gadget Controller"
+	depends on ARCH_AT91
+
 config USB_GADGET_DWC2_OTG
 	bool "DesignWare USB2.0 HS OTG controller (gadget mode)"
 	select USB_GADGET_DUALSPEED