ARM: sun8i: a83t: cubietruck-plus: Enable SPDIF output

The Cubietruck Plus has an optical SPDIF out connector.
Enable SPDIF audio output for this board.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>