Correctly handle nodes compatible with "syscon", "simple-bus".  Syscon can
also have child nodes that share a registration file with it.  To do this
correctly, follow these steps: - subclass syscon from simplebus and expose it
if the node is also
  "simple-bus" compatible.
- block simplebus probe for this compatible string, so it's priority
 (bus pass) doesn't colide with syscon driver.