arm64: dts: allwinner: a64: Add SPDIF to the Pine64

The S/PDIF transmitter can be reached on the Euler connector.
But as this is a GPIO then leave it disabled so that an overlay
can override the status property.

Signed-off-by: Marcus Cooper <codekipper@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>