MFC r275779, r275963, r276101, r276161, r276297:
Move ofw_cpu.c to sys/dev/ofw so that it can be used by other architectures. Add driver for CPU frequency/voltage control on the Raspberry Pi. On initialization, do not use bcm_mbox_intr() to read the pending messages. This fixes the hang that happens on boot while initializing the cpufreq on Raspberry Pi.