gpiobus: add a gpiobus_add_bus function

Some drivers need to postpone the attachment of gpiobus until hardware
init is done. Add a new gpiobus_add_bus function to accommodate this
case.

Suggested by: mmel, andrew
Reviewed by: mmel, imp, andrew
Approved by: imp (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D51133