ASoC: ADS117x ADC driver

This patch adds support for the TI ADS117x family of multichannel ADCs
and was sponsored by Shotspotter Inc.

Signed-off-by: Graeme Gregory <gg@slimlogic.co.uk>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>