This adds a driver that implements the greybus Raw protocol as
specified.
It preserves the message boundries by only allowing a read to receive
a "full" message, and any write() call also is passed in a single
greybus request.
Totally untested, given that we have no raw firmware or gbsim code yet.
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com> Reviewed-by: Rui Miguel Silva <rui.silva@linaro.org>