[POWERPC] Celleb: Support for Power/Reset buttons

This supports Power/Reset buttons on Beat on Celleb.

On Beat, we have an event from Beat if Power button or Reset button
is pressed. This patch catches the event and convert it to a signal
to INIT process by calling ctrl_alt_del() function.

/sbin/inittab have no entry to turn the machine power off so we have
to detect if power button is pressed or not internally in our driver.
This idea is taken from PS3's event handling subsystem.

Signed-off-by: Kou Ishizaki <Kou.Ishizaki@toshiba.co.jp>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>