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>
diff --git a/drivers/power/reset/Kconfig b/drivers/power/reset/Kconfig
index 5cec1d9..1131cf7 100644
--- a/drivers/power/reset/Kconfig
+++ b/drivers/power/reset/Kconfig
@@ -15,7 +15,7 @@
 	  This driver supports turning off board via a ams AS3722 power-off.
 
 config POWER_RESET_AT91_POWEROFF
-	bool "Atmel AT91 poweroff driver"
+	tristate "Atmel AT91 poweroff driver"
 	depends on ARCH_AT91
 	default SOC_AT91SAM9 || SOC_SAMA5
 	help