MFC r308767 by br:

Make gpiobus early driver at BUS_PAS_BUS.

The gpiobus driver is attached explicitly and generally should be
at the same pass as its parent. Making it use BUS_PAS_BUS ensures
that it attaches immediately after parent adds it (assuming the
parent itself attached at BUS_PAS_BUS and above).

Submitted by: kan
Sponsored by: DARPA, AFRL