ARM: highbank: handle soft poweroff and reset key events

Graceful reboot and poweroff via IPMI commands to the management
processor don't work. Power and reset keys are events from the
management processor which are generated via IPC messages. Passing
the keys to userspace does not work as neither acpid nor a desktop
environment are present.

This adds a notifier handler for the IPC messages so the kernel can
handle the key events directly and IPMI graceful shutdown will work.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Cc: stable@vger.kernel.org
Signed-off-by: Olof Johansson <olof@lixom.net>