arm64: dts: allwinner: Add SPDIF node for Allwinner H6

The Allwinner H6 has a SPDIF controller called OWA (One Wire Audio).

Only one pinmuxing is available so set it as default.

Signed-off-by: Clément Péron <peron.clem@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>