greybus: gpbridge: Add gpbridge driver init/exit helper macros

In order to help bridged-phy drivers to define init()/exit()
functions, define helper macro in gpbridge.h file.

Testing Done: Tested on gbsim.

Signed-off-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Tested-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>