powerpc/powernv: reboot when requested by firmware

Use orderly_reboot so userspace will to shut itself down via the reboot
path.  This is required for graceful reboot initiated by the BMC, such as
when a user uses ipmitool to issue a 'chassis power cycle' command.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Cc: Fabian Frederick <fabf@skynet.be>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Jeremy Kerr <jk@ozlabs.org>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>