ARM: at91: at91_dt_defconfig: add back USB gadget FS

The older USB gadget FS defconfig option was removed whereas it's still
in use for early ARM926 SoCs. Add it back as a module.

Reported-by: Larry Gass <larry.gass@microchip.com>
Fixes: 6094964 ("ARM: at91: at91_dt_defconfig: update")
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>

Author: noglitch

arch/arm/configs/at91_dt_defconfig

@@ -176,6 +176,7 @@ CONFIG_USB_SERIAL_GENERIC=y
 CONFIG_USB_SERIAL_FTDI_SIO=y
 CONFIG_USB_SERIAL_PL2303=y
 CONFIG_USB_GADGET=y
+CONFIG_USB_AT91=m
 CONFIG_USB_ATMEL_USBA=m
 CONFIG_USB_CONFIGFS=m
 CONFIG_USB_CONFIGFS_ACM=y