ASoC: audio-graph-card: add ep_to_port() / port_to_ports()

Current audio-graph-card is using of_get_parent() to get "port" from
"ep", or get "ports" from "port". But it is difficlut to understand,
and "ports" might not exist.
This patch adds ep_to_port() to get "port" from "ep", and port_to_ports()
to get "ports" from "port". "ports" will be NULL if not exist.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://msgid.link/r/8734q2h6fz.wl-kuninori.morimoto.gx@renesas.com
Signed-off-by: Mark Brown <broonie@kernel.org>