greybus: Audio: Add skeleton code for GB virtual codec driver

This patch adds gb-codec driver with static information for
DAPM widgets, controls & dapm_routes.

Including some changes in kernel code(machine driver):
- Able to register codec and glue it with existing sound card successfully.
- Able to view & modify mixer controls:
        (volume/mute[left/right][input/output])
- Able to view DAPM widgets registered via /debug interface.
- Able to establish DAPM path for playback.

Since, FE<->BE path not yet verified with default jetson build,
registering GB DAI as normal DAI link to verify GB virtual codec
specific DAPM path.

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