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: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Marek Vasut <marex@denx.de>