ASoC: Document wm8994 endpoints

The wm8994 exposes several inputs and outputs that can be used by
machine drivers in their routing.  Add them to the documention so
they don't have been duplicated in any machine drivers bindings.

Signed-off-by: Jonathan Bakker <xc-racer2@live.ca>
Link: https://lore.kernel.org/r/BN6PR04MB066019A8783D22F1C4A588B7A39F0@BN6PR04MB0660.namprd04.prod.outlook.com


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