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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