ASoC: dt-bindings: everest,es8328: Document audio graph port

The ES8328/ES8388 audio codec is currently used in conjunction with
audio-graph-card to provide an endpoint for binding with the other side
of the audio link.

This is achieved via the 'port' property, which is not supported by the
binding:

rk3588s-indiedroid-nova.dtb: audio-codec@11: 'port' does not match any of the regexes: 'pinctrl-[0-9]+'
from schema $id: http://devicetree.org/schemas/sound/everest,es8328.yaml#

Document the missing property.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://patch.msgid.link/20241019-es8328-doc-port-v1-1-25c1d1b5c65c@collabora.com
Signed-off-by: Mark Brown <broonie@kernel.org>