Make simplebus a base class of ofwbus. This allows the elimination of
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