greybus: gpbridge: Export few routines

In order to separate protocol specific drivers into their own modules,
some of the gpbridge routines need to be exported.

Tested on EVT 1.5 by inserting GP test module, all the devices were
enumerated correctly.

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