This is a mess:

-> sys/mutex.h -> sys/intr.h -> machine/intr.h -> footbridge/footbridge_intr.h
-> arm/cpu.h -> sys/cpu_data.h -> sys/sched.h -> sys/time.h -> sys/timevar.h
-> sys/systm.h -> sys/param.h -> uvm_param.h -> sys/resourcevar.h

Now sys/resourcevar.h needs a concrete definition of kmutex_t, and
although we started including sys/mutex.h we never got to include
machine/mutex.h which actually defines it, since the footbridge_intr.h
took us for a long ride. Take the easy way out and include
arm/mutex.h in footbridge_intr.h so that we get the definition we
need.

(christos)