Attach this driver during BUS_PASS_BUS and move the cpu init code to a
bus_new_pass() handler so it doesn't happen until BUS_PASS_CPU. This allows
the anatop driver to outbid the generic simplebus driver (which the FDT
data describes as compatible).

Some day when we handle power regulators, this driver may actually
become a functional simplebus and attach the regulators as children, as
described in the FDT data.