ASoC: ak5558: Add support for AK5558 ADC driver

AK5558 is a 32-bit, 768 kHZ sampling, differential input ADC
for digital audio systems.

Datasheet is available at:

https://www.akm.com/akm/en/file/datasheet/AK5558VN.pdf

Initial patch includes support for normal and TDM modes.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Junichi Wakasugi <wakasugi.jb@om.asahi-kasei.co.jp>
[initial coding for 3.18 kernel]
Signed-off-by: Mihai Serban <mihai.serban@nxp.com>
[cleanups and porting to 4.9 kernel]
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
[tdm support]
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
[pm support, cleanups and porting to latest kernel]
Signed-off-by: Mark Brown <broonie@kernel.org>