power: reset: Add AT91 poweroff driver

Add a driver to handle the shutdown of the Atmel SoCs. This code used to be
(and still is) in arch/arm/mach-at91. We didn't remove it yet so that we can
convert all the boards to using this driver, before removing it entirely in a
separate patch.

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