mfd: ab8500-sysctrl: Fix sparse warning

Fix sparse warning:

drivers/mfd/ab8500-sysctrl.c:26:6: warning: symbol 'ab8500_power_off' was not declared. Should it be static?

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>