ASoC: atmel: machine driver for at91sam9x5-wm8731 boards

Description of the Asoc machine driver for an at91sam9x5 based board
with a wm8731 audio DAC. Wm8731 is clocked by a crystal and used as a
master on the SSC/I2S interface. Its connections are a headphone jack
and an Line input jack.

[Richard: this is based on an old patch from Nicolas that I forward
ported and reworked to use only device tree]

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>