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.

Arm platforms which use FDT data to enumerate devices have been relying
on devices being attached in the exact order they're listed in the dts
source file.  That's one of things currently preventing us from using
vendor-supplied fdt data (because then we don't control the order of the
devices in the data).  Multi-pass attachment can go a long way towards
solving that problem by ensuring things like clock and interrupt drivers
are attached before the more mundane devices that need them.

The long-term goal is to have all arm fdt-based platforms using multipass.
This option is a bridge to that, letting us enable it selectively as
platforms are converted and tested (the alternative being to just throw
a big switch and try to fight fires as they're reported).

sys/arm/arm/nexus.c

@@ -125,7 +125,12 @@ static driver_t nexus_driver = {
 	nexus_methods,
 	1			/* no softc */
 };
+#ifdef ARM_DEVICE_MULTIPASS
+EARLY_DRIVER_MODULE(nexus, root, nexus_driver, nexus_devclass, 0, 0, 
+    BUS_PASS_BUS + BUS_PASS_ORDER_EARLY);
+#else
 DRIVER_MODULE(nexus, root, nexus_driver, nexus_devclass, 0, 0);
+#endif
 
 static int
 nexus_probe(device_t dev)

sys/conf/options.arm

@@ -1,6 +1,7 @@
 #$FreeBSD$
 ARM9_CACHE_WRITE_THROUGH	opt_global.h
 ARM_CACHE_LOCK_ENABLE	opt_global.h
+ARM_DEVICE_MULTIPASS	opt_global.h
 ARM_KERN_DIRECTMAP	opt_vm.h
 ARM_L2_PIPT		opt_global.h
 ARM_MANY_BOARD		opt_global.h