configs: Enable CONFIG_ARM_BCM2835_CPUFREQ

We actually use the 64-bit 2711 kernel on Pi3 too.
It still uses the downstream cpufreq driver in DT.

See: https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=269769&start=100#p1644752


Signed-off-by: popcornmix <popcornmix@gmail.com>