greybus: audio: Add module specific driver

Use seperate driver to process GB Audio modules plugged-in.
It'll use helper function register_module to attach itself
to gbaudio-codec driver.

Signed-off-by: Vaibhav Agarwal <vaibhav.agarwal@linaro.org>
Reviewed-by: Mark Greer <mark.greer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>