Pretty much any other device might need to manipulate a gpio pin during its probe or attach routines, so these devices must be available as early as possible. The gpio device is an interrupt controller, but I didn't choose the INTERRUPT pass for that reason (it works fine as an interrupt controller as long as it attaches any time before interrupts are enabled). That just looked like the right place in the passes to ensure that it attaches before any type of device that might need gpio pin manipulations.
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