Create a mechanism for looking up a device_t associated with an ofw/fdt
xref handle, and for registering that association.  Also use the same data
for faster translations between node and xref handles.

Add OF_xref_from_device() so that there's no need to have an intermediate
call to ofw_bus_get_node() to lookup info that's already in the xreflist.

When registering an association between a device and an xref phandle, create
an entry in the xref list if one doesn't already exist for the given handle.