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