greybus: host: provide "generic" apbridge output calls

Provide a new function, gb_hd_output() to send data to the apbridge.
This is useful for the camera and audio drivers that need to do this
type of messaging.

The camera driver is converted to use this new function, the audio
driver can use it when it gets merged later.

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