doc: dts: Add device tree information regarding wm8940 codec

This commit provides documentation entry for wm8940 codec.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20220509121313.31216-1-lukma@denx.de


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