arm64: 'reboot' doesn't work, needs to pull the plug

[Impact]
The 'reboot' command doesn't work in the arm64 variant of the raspi2 kernel: after issuing 'reboot' the board starts the reboot process, reaches its end and prints '[ 451.761674] reboot: Restarting system' but then it sits there forever - the only way to reboot the board is to phisically pull the plug. It only affects Yakkety.

After some investigation i found what's going on: the reboot process for the armhf and arm64 variants of the raspi2 kernel is significanly different, even if it uses the same hardware mechanism.

On armhf, in the board code (arch/arm/mach-bcm2709/bcm2709.c::bcm2709_restart()) that is executed early during boot, bcm2709_restart() is registered as the restart callback in the board data structure, and whenever we execute 'reboot' the function is called: it directly initializes the watchdog hardware with a very short timeout, kicks it and then sits there waiting for the timer to expire (and the watchdog to reboot the board)

For arm64, on the other hard, there's no board code (the board code is a relic from the pre-dt period and when arm64 was started they went DT only from the get go), so there isn't a board structure containing custom functions for every board (init board call back, post init callback, restart call back, etc), instead the arm64 reboot code (arch/arm64/kernel/process.c::machine_restart()) invokes the generic reboot code (kernel/reboot.c::do_kernel_restart()) which in turn walks and invokes, every reboot handler that was registered on the restart_handler_list (kernel/reboot::restart_handler_list) - in other words, it relies on another piece of code (able to reset the board) to register its reboot function on that handler list and then it uses it. In the raspberry board, the hardware capable of rebooting the board is the hardware watchdog (that among the other things register a reboot handler once it attaches), but on Y/arm64 the kernel driver for such hardware was built as a module and it doesn't autoload on boot, so restart_handler_list results empy when 'reboot' is invoked.

[Fix]
Built-in the watchdog driver (CONFIG_BCM2835_WDT).

[Test case]
Try to execute 'reboot' on Y/arm64 and watch the board sits at the 'Restarting system', then install a kernel with the watchdog driver built-in and execute reboot again.