greybus: Add control protocol support

Add control protocol driver that is responsible for handling operations
on control CPort. The AP also needs to support incoming requests on its
control port. Features not implemented yet are marked as TODO for now.

NOTE: This also fixes cport-bundle-id to 0 and cport-id to 2 for control
protocol.

Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>