[rpi] Fix compatiblity with upstream DTB for RPi 3B and 3B+

Upstream dtb switched to using brcm,bcm2837 for platform
compatibility string. Patch platfrom and cpufreq compatiblity
data accordingly.

Submitted by: Sylvain Garrigues <sylgar@gmail.com>
Tested by: db@
Differential Revision: https://reviews.freebsd.org/D15998