ASoC: cs42l51: add multi endpoint support

Support multiple endpoints on cs42L51 codec port
when used in of_graph context.

This patch allows to share the codec port between two CPU DAIs.

Example:

STM32MP157C-DK2 board uses CS42L51 audio codec.
This codec is connected to two serial audio interfaces,
which are configured either as rx or tx.

From AsoC point of view the topolgy is the following:
// 2 CPU DAIs (SAI2A/B), 1 Codec (CS42L51)
Playback: CPU-A-DAI(slave) -> (master)CODEC-DAI/port0
Record:   CPU-B-DAI(slave) <- (master)CODEC-DAI/port0

In the DT two endpoints have to be associated to the codec port:
cs42l51_port: port {
	cs42l51_tx_endpoint: endpoint@0 {
		remote-endpoint = <&sai2a_endpoint>;
	};
	cs42l51_rx_endpoint: endpoint@1 {
		remote-endpoint = <&sai2b_endpoint>;
	};
};

However, when the audio graph card parses the codec nodes, it expects
to find DAI interface indexes matching the endpoints indexes.

The current patch forces the use of DAI id 0 for both endpoints,
which allows to share the codec DAI between the two CPU DAIs
for playback and capture streams respectively.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>