alpha: fix reboot on Avanti platform

We need to define NEED_SRM_SAVE_RESTORE on the Avanti, otherwise we get
machine check exception when attempting to reboot the machine.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Matt Turner <mattst88@gmail.com>