ARM: sun8i: a83t: Add device node for SPDIF transmitter

The A83T SoC has an SPDIF transmitter block. According to the vendor
BSP kernel, it is compatible with the one found on the H3 SoC.

Add a device node and pinmux setting for it.

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