ASoC: sgtl5000: Add audio-graph-card port

The SGTL5000 codec can be connected via audio-graph-card,
add the missing port: entry into the bindings.

Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20210610150135.29905-1-marex@denx.de


Signed-off-by: Mark Brown <broonie@kernel.org>