RFC: Common machine reset handling

Hi,

I've been looking for a solution to my bcm4760 watchdog based restart
hook when I noticed that the kernel reboot/shutdown mechanism is having a few unaddressed issues.

Those I identified are:

1) context pointer often needed by the reset hook (currently local static data is used for this pourpose)
2) unclear ownership/policy in case of multiple reset hooks (currently almost nobody seems to care much)

I put together this patchset to explore some possible interface and improve the status, essentially moving away from the function-pointer-assignament kind of reset hook registration.

It is meant to be platform agnostic and sits in kernel/machine_reset.c. It opts in only when needed and when supported.

A common usage, as reset hook provider (drivers, boardfiles):

void restart_hook(void *dev, enum reboot_mode mode, const char *cmd)

{
	/* reboot the board */
}

void release_hook(void *dev)

{
	/* release the hook */
}

void init_restart_hook(void *dev)

{
	struct reset_hook hook;
	reset_hook_init(&hook);
	hook.restart = restart_hook;
	hook.release = release_hook; /* optional */
	set_machine_reset(RESET_RESTART, &hook, dev);
}

A common usage, as reset hook consumer (arch specific hooks, kernel):

void machine_restart(char *command)

{
	if (_machine_restart)
		_machine_restart(command);
	else
		default_restart(reboot_mode, command);
}

void machine_halt(void)

{
	if (_machine_halt)
		_machine_halt();
	else
		default_halt();
}

void machine_power_off(void)

{
	if (pm_power_off)
		pm_power_off();
	else
		default_power_off();
}

I don't see these impressive diffstats but at least it's possible to free some static data along the way. Maybe there are some other desiderata that could be satisfied?

Comments are welcome.

Thanks,
Domenico