Fix cpufreq(4) on RPI-B

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