SMP support for Raspberry Pi 3

[Oleksandr Tymoshenko]

Hello,

SMP support for Raspberry Pi 3 is in usable state now. I didn’t run
any load tests, only checked that all four cores are started on boot.

Short instruction:

- Build GENERIC arm64 kernel for your image, not GENERIC-UP
- Copy https://people.freebsd.org/~gonzo/arm/rpi3-smp/u-boot.bin and
    https://people.freebsd.org/~gonzo/arm/rpi3-smp/armstub8.bin to FAT
    partition on SD card.
- Edit config.txt: change value of device_tree_address parameter:
    device_tree_address=0x4000
- Reboot device. It should boot with all four cores enabled and reboot
    should work too.

More detailed description of what all these binaries/changes contain
is in these two posts:

https://kernelnomicon.org/?p=714
https://kernelnomicon.org/?p=718

Please note that FreeBSD/arm64 has been broken by clang 3.9.0 import
and you will have to update aarch64-binutils to the 2.27_5 to get
it booting. Details: https://reviews.freebsd.org/D8622

-- 
gonzo