ARM: at91: Register the poweroff driver

Register the poweroff driver in the old-style SoC definition so that the driver
is loaded and provide a shutdown hook.

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>