power/reset: at91-poweroff: allow compiling as a module

It was not possible to compile at91-poweroff as a module. Implement
.remove() to allow it. Also switch to module_platform_driver_probe() as it
is not hotpluggable.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>