[rpi] rpi3 should use the same cpufreq logic as rpi2, not rpi-b

RPi3 cpufreq is more like that on RPi2. Setting arm frequency
above min (say, "sysctl hw.cpufreq.arm_freq=600000001") turns on
turbo mode, and the firmware automatically raises voltage, sets
frequency to max 1200MHz, and throttle when overheat, etc.

Swap if/else parts and use SOC_BCM2835 def so RPi3 can share the
same cpufreq logic as RPi2, instead of falling to that for RPi.

Submitted by: Jia-Shiun Li <jiashiun@gmail.com>
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D9640