greybus: gpbridge.h: move protocol init/exit prototypes

Create gpbridge.h for the gpbridge-specific function prototypes, the
rest of the greybus drivers don't care about them.

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