Set ofwbus and simplebus to attach during BUS_PASS_BUS.

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

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.

Attach arm generic interrupt and timer drivers in the middle of
BUS_PASS_INTERRUPT and BUS_PASS_TIMER, respectively.

Add an arm option, ARM_DEVICE_MULTIPASS, used to opt-in to multi-pass
device attachment on arm platforms.  If this is defined, nexus attaches
early in BUS_PASS_BUS, and other busses and devices attach later, in the
pass number they are set up for.  Without it defined, nexus attaches in
BUS_PASS_DEFAULT and thus so does everything else, which is status quo.