mfd: ab8500-sysctrl: Always enable pm_power_off handler

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: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>