ARM: at91: Remove poweroff DT probing

Now that the poweroff code is a driver of its own, remove the DT probing in
mach-at91 and let the usual DT code do its job.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>