pinctrl: sun6i: add SPDIF to pin description.

Add the SPDIF mux functionality to the pinctrl for the Allwinner A31
SoC.

Signed-off-by: Marcus Cooper <codekipper@gmail.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>