Use ofw_bus_find_child_device_by_phandle to see if the child we're
adding already exists and if so just return that. The typical use case
is from identify routines, which shouldn't be adding multiple copies
of the same phandle_t to the gpiobus. Only one per phandle_t is needed
(or expected by the current code).

Differential Revision: https://reviews.freebsd.org/D2871