arm64: dts: meson-sm1: add spdifin and pdifout nodes

Add spdifin spdifout nodes for Amlogic SM1 SoCs.

Signed-off-by: Artem Lapkin <art@khadas.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
[narmstrong: subject and DT fixups from mailing-list review]
Link: https://lore.kernel.org/r/20211215030236.340841-1-art@khadas.com