staging: greybus: Add remaining audio files to maintained list

Some Greybus Audio Device Class Protocol driver files are missing
in the MAINTAINERS file even though they are maintained so add
them.

CC: Vaibhav Agarwal <vaibhav.sr@gmail.com>
Signed-off-by: Mark Greer <mgreer@animalcreek.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>