mfd: Make AB8500_CORE select POWER_SUPPLY to fix build error

This patch fixes below build error when CONFIG_POWER_SUPPLY is not set.

drivers/built-in.o: In function `ab8500_power_off':
drivers/mfd/ab8500-sysctrl.c:37: undefined reference to `power_supply_get_by_name'
drivers/mfd/ab8500-sysctrl.c:53: undefined reference to `power_supply_get_by_name'
make: *** [vmlinux] Error 1

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>