mfd: Arizone core should select MFD_CORE

Otherwise, with:

CONFIG_MFD_ARIZONA=y
CONFIG_MFD_ARIZONA_I2C=m
CONFIG_MFD_CORE=m

We get:

drivers/built-in.o: In function `arizona_dev_init':
(.devinit.text+0x3ab0): undefined reference to `mfd_add_devices'
drivers/built-in.o: In function `arizona_dev_init':
(.devinit.text+0x3fdc): undefined reference to `mfd_add_devices'
drivers/built-in.o: In function `arizona_dev_init':
(.devinit.text+0x3fff): undefined reference to `mfd_add_devices'
drivers/built-in.o: In function `arizona_dev_init':
(.devinit.text+0x4059): undefined reference to `mfd_remove_devices'
drivers/built-in.o: In function `arizona_dev_exit':
(.devexit.text+0x9): undefined reference to `mfd_remove_devices'

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>