greybus: raw: add raw greybus kernel driver

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>