MFC r360779: gpioiic_attach: fix a NULL pointer crash on hints-based systems

The attach method uses GPIO_GET_BUS() to get a "newbus" device
that provides a pin. But on hints-based systems a GPIO controller
driver might not be fully initialized yet and it does not know gpiobus
hanging off it. Thus, GPIO_GET_BUS() cannot be called yet.
The reason is that controller drivers typically create a child gpiobus
using gpiobus_attach_bus() and that leads to the following call chain:
gpiobus_attach_bus() -> gpiobus_attach() ->
bus_generic_attach(gpiobus) -> gpioiic_attach().
So, gpioiic_attach() is called before gpiobus_attach_bus() returns.

I observed this bug with nctgpio driver on amd64.
I think that the problem was introduced in r355276.

The fix is to avoid calling GPIO_GET_BUS() from the attach method.
Instead, we know that on hints-based systems only the parent gpiobus can
provide the pins.
Nothing is changed for FDT-based systems.