Add a set of helpers (ofw_bus_get_status() and ofw_bus_status_okay()) to
process "status" properties of OF nodes.

I've avoided adding new KOBJ methods here so that we don't have to modify
every ofw_bus in the tree. Since 100% of implementations of ofw_bus use
only ofw_bus_gen_*(), it might be worth garbage-collecting the other
methods as well.