ARM: mx6: Enable STDIO deregistering on Novena

Novena supports USB keyboard, which is a pluggable device and can be
unplugged. Thus, we need to be able to deregister it's stdio device.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
diff --git a/include/configs/novena.h b/include/configs/novena.h
index cfb92d6..e938fbc 100644
--- a/include/configs/novena.h
+++ b/include/configs/novena.h
@@ -150,6 +150,7 @@
 #define CONFIG_USB_EHCI_MX6
 #define CONFIG_USB_STORAGE
 #define CONFIG_USB_KEYBOARD
+#define CONFIG_SYS_STDIO_DEREGISTER
 #define CONFIG_SYS_USB_EVENT_POLL_VIA_CONTROL_EP
 #define CONFIG_USB_HOST_ETHER
 #define CONFIG_USB_ETHER_ASIX