nios2: permit device tree control of U-Boot

Enable this in the Kconfig so that nios2 boards can use device
tree to configure U-Boot.
Signed-off-by: NThomas Chou <thomas@wytron.com.tw>
Reviewed-by: NSimon Glass <sjg@chromium.org>
Acked-by: NMarek Vasut <marex@denx.de>

arch/Kconfig

@@ -63,6 +63,7 @@ config NIOS2
 	bool "Nios II architecture"
 	select HAVE_GENERIC_BOARD
 	select SYS_GENERIC_BOARD
+	select SUPPORT_OF_CONTROL
 
 config OPENRISC
 	bool "OpenRISC architecture"