Kconfig: fix undefined symbols (g_dnl*) when NET_LWIP is default enabled

Fix for link errors on am62px_evm_a53and other platforms when NET_LWIP
is enabled:

 common/dfu.c:34:(.text.run_usb_dnl_gadget+0x68): undefined reference to `g_dnl_clear_detach
 [...]
 common/spl/spl_dfu.c:29:(.text.spl_dfu_cmd+0xb0): undefined reference to `run_usb_dnl_gadget'

- DFU_OVER_USB compiles common/dfu.c which calls g_dnl_clear_detach()
which is implemented in drivers/usb/gadget/g_dnl.c which needs
USB_GADGET_DOWNLOAD
- SPL_DFU compiles common/spl/spl_dfu.c which calls run_usb_dnl_gadget()
which is implemented in common/dfu.c which needs DFU_OVER_USB

Therefore add these dependencies to Kconfig.

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>