greybus: APBridge: move APBridge request protocol to a common .h file

This moves all of the APBridge request protocol commands that are
currently used to a common .h file for everyone to be able to use them
in the future, where needed.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Tested-by: Mark Greer <mgreer@animalcreek.com>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>