gpio: attach gpiobus when the controller is ready

Only attach gpiobus when the controller is fully initialized. Children
of gpiobus expect this to be the case.

Reviewed by: mmel, imp, andrew
Approved by: imp (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D51088

(cherry picked from commit 4695e3aa7c685c092cb4b2662bee16c31be790f8)