[ARM] Orion: Add Buffalo Linkstation Mini power-off method

This patch adds specific power-off method for Buffalo Linkstation Mini
board. The board has a hardware switch which should be monitored from
userspace. When the switch is in OFF position the board should be rebooted
and U-Boot will start in an idle mode and wait for the user to move the
power switch back to ON position.

Signed-off-by: Alexey Kopytko <alexey@kopytko.ru>
Signed-off-by: Nicolas Pitre <nico@marvell.com>