ARM: mvebu: Add DT entry for ReadyNAS Duo v2 to use gpio-poweroff driver

Without that fix, at the end of the shutdown process, the board is
still powered (led glowing, fan running, ...).

Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>