lsxl: support power switch

This patch restores the Linkstation's original behaviour when powering off.
Once the (soft) power switch is turned off, linux will reboot and the
bootloader turns off HDD and USB power. Then it loops as long as the switch
is in the off position, before continuing the boot process again.

Additionally, this patch fixes the board function set_led(LED_OFF).

Signed-off-by: Michael Walle <michael@walle.cc>
Cc: Prafulla Wadaskar <prafulla@marvell.com>