gpio: remove redundant calls to bus_attach_children

gpiobus_attach_bus already calls it.

Reviewed by: mmel, imp, andrew
Approved by: imp (mentor)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D51108

(cherry picked from commit c2d8cca77152ebf72f488490163585be7e34eb0f)