duplicated code in the two classes, and also allows devices in FDT-based
systems to declare simplebus as their parent and still work correctly
when the FDT data describes the device at the root of the tree rather
than as a child of a simplebus (which is common for interrupt, clock,
and power controllers).
Differential Revision: https://reviews.freebsd.org/D1990
Submitted by: Michal Meloun
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