ASoC: Improve hi6210-i2s DT bindings

This patch improves the previously submitted hi6210-i2s DT
binding, adding extra details to how the multi-dai index
value maps to the potential interfaces.
(Currently just index 0 -> the S2 interface, as there is
only one supported, but in the future other interfaces may
be enabled.)

Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>