ASoC: audio-graph: Support empty Codec endpoint

For open platforms, which can support pluggable audio cards, Codec
endpoint is not fixed always. It actually depends on the compatible
HW module that is going to be connected. From SoC side the given I/O
interface is always available. Hence such links have fixed CPU endpoint
but no Codec endpoint. This patch helps to support such links where
user can populate Codec endpoint only and its fields in Platform DT
depending on the plugged HW.

Signed-off-by: Sameer Pujar <spujar@nvidia.com>
Cc: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Link: https://lore.kernel.org/r/1604329814-24779-6-git-send-email-spujar@nvidia.com


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