x86: add ACPI reboot option

Add the ability to reboot an x86_64 based machine using the RESET_REG in the
FADT ACPI table.

Signed-off-by: Aaron Durbin <adurbin@google.com>
Cc: Len Brown <lenb@kernel.org>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>