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.

While I'm in, also block "syscon", "simple-mfd" for the same reason.

MFC after:	4 weeks

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