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

[manu]

Summary

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

MFC after: 1 month
Sponsored by: Diablotin Systems

Test Plan

root@nanopi-m1-plus:~ # sysctl hw.fdt
hw.fdt.freebsd-version: 5.13
hw.fdt.compatible: friendlyarm,nanopi-m1-plus allwinner,sun8i-h3
hw.fdt.model: FriendlyArm NanoPi M1 Plus

root@:/ # sysctl hw.fdt
hw.fdt.freebsd-version: 5.13
hw.fdt.compatible: pine64,sopine-baseboard pine64,sopine allwinner,sun50i-a64
hw.fdt.model: SoPine with baseboard

[imp]

Very nice. I've had a hack that's a crude version of this in my tree from a while ago, but had let it go stale for lack of time to do all the things I want.