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

sys/arm/broadcom/bcm2835/bcm2835_cpufreq.c

@@ -126,6 +126,7 @@ static struct ofw_compat_data compat_data[] = {
 	{ "broadcom,bcm2835-vc",	1 },
 	{ "broadcom,bcm2708-vc",	1 },
 	{ "brcm,bcm2709",	1 },
+	{ "brcm,bcm2835",	1 },
 	{ "brcm,bcm2836",	1 },
 	{ "brcm,bcm2837",	1 },
 	{ NULL, 0 }