This patch adds support for the wm8581 codec to the wm8580 driver.
The wm8581 codec hardware adds a fourth DAC and otherwise is
compatible with the wm8580 codec.

of_device_id data is used to allow the driver to select the
suitable DAC count specified in the device tree codec selection.
The wm8580_driver_data struct is used to store the number of DACs.

The snd_soc_dai_driver no longer lists the channels_max for the
playback substream. This variable is set during the i2c probe
from the of_device_id supplied wm8580_driver_data struct.

With knowledge of the number of DACs in use, the DAC4 controls,
widgets and routes are added as required for DAC4.

The device tree documentation for the wm8580 is altered to list
the wm8581 codec support, as is the Kconfig file.
Signed-off-by: Matt Flax <flatmax@flatmax.org>
Reviewed-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>