greybus: protocol: Add gb_gpbridge_protocol_driver()

There are many gpbridge protocol drivers that need gb_protocol_driver()
without the module_init/exit() lines. Lets create one for them.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>