AB8500 sysctrl driver implements a pm_power_off handler, but that is
currently not registered until a specific platform data field is
enabled.

This patch drops the platform data field and always registers
ab8500_power_off if no other pm_power_off handler was defined before,
and also introduces the necessary cleanup code in the driver's remove
function.
Acked-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NFabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>