greybus: Initial I2S definitions

These are definitions from Mark that I've consolidated into
one header file. I'd like to get these merged at some point
soon, so the audio driver and gbsim work can avoid having
out-of-tree dependencies.

Signed-off-by: Mark A. Greer <mgreer@animalcreek.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>