greybus: audio: Use single codec driver registration

We have single I2S port via APB1 for communication with all
audio modules. Thus, we should register single codec driver
and manage all individual audio modules internally within
this 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>