fdt: Expose the model, compatible and freebsd dts brandind as sysctl

This make it easier for script to get the hardware on which they are running.

Sponsored by: Diablotin Systems
Differential Revision: https://reviews.freebsd.org/D31205
Reviewed by: imp
Should be ok on powerpc: jhibbits (over irc)

(cherry picked from commit 50e0dc0c4b46ee62b898ce2d92e52be4f77383d9)