rather than relying on a set of canned EARLY_DRIVER_MODULE() statements in
the ofw_iicbus source. This means hw drivers will no longer be required to
use one of a few predefined driver names. They will also now be able to
decide themselves if they want to use DRIVER_MODULE or EARLY_DRIVER_MODULE
and to set which pass to attach on for early modules.
Mainly, this adds extern declarations for the driver and devclass variables.
It also renames ofwiicbus_devclass to ofw_iicbus_devclass to be consistant
with the way we use ofw_ prefixes on this stuff.
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