gpio: remove gpiobus_attach_bus

Since gpiobus_attach_bus can attach the gpiobus child along with its
children in the same bus pass, the parent controller's reference to
gpiobus might not be set by t

gpio: remove gpiobus_attach_bus

Since gpiobus_attach_bus can attach the gpiobus child along with its
children in the same bus pass, the parent controller's reference to
gpiobus might not be set by the time the children need it. Instead,
drivers should use gpiobus_add_bus and explicitly call
bus_attach_children.

Reviewed by: mmel, imp (older version)
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D51578

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