Define names that drivers can use to adjust their position relative to
other drivers within a BUS_PASS.

Reviewed by:	imp
Adjust ofwbus and simplebus to attach at BUS_PASS_ORDER_MIDDLE, so that
a platform can attach some other bus first if necessary.
Set the pl310 L2 cache driver to attach during the middle of BUS_PASS_CPU.
Because that's earlier than interrupts are available, set up deferred
configuration of interrupts (which are used only for debugging).