[IA64-SGI] Altix: enable poweroff

[IA64-SGI] Altix: enable poweroff

This patch adds the necessary "hook" to allow SGI/SN
machines to perform a system power off upon a
'init 0', 'halt -p', 'poweroff' or 'shutdown -h'.

The "hook" is to set the pm_power_off callback
to ia64_sn_power_down(). pm_power_off is checked
in machine_power_off()/do_poweroff() and, if set, is executed.
ia64_sn_power_down() is a function already present (but not
used currently) in the sn kernel.
ia64_sn_power_down() makes a SAL call to execute the
power off.

Signed-off-by: Aaron J Young <ayoung@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>