mfd: motorola-cpcap: Add audio-codec support

Add support for the audio-codec node by converting from
devm_of_platform_populate() to devm_mfd_add_devices().

Tested-by: Pavel Machek <pavel@ucw.cz>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Lee Jones <lee.jones@linaro.org>