Since r324184 the root node compatible for the original Raspberry Pi
is "brcm,bcm2835", add it to the compatible list of bcm2835_cpufreq.
Tested On: RPi1 Model B
Note that the default Das U-Boot FDT does not include a cpus clause
so actually adding a bcm2835_cpufreq device requires adding a FDT
overlay defining the cpu.
Approved by: grog, jhb (mentors)
MFC after: 3 days