mfd: tps65912: Select MFD_CORE

CONFIG_MFD_CORE must be selected for TPS65912 to properly buid.
Otherwise it results in a link error:

	drivers/built-in.o: In function `tps65912_device_init':
	(.text+0x587e4): undefined reference to `mfd_add_devices'
	drivers/built-in.o: In function `tps65912_device_init':
	(.text+0x5884c): undefined reference to `mfd_remove_devices'
	drivers/built-in.o: In function `tps65912_device_exit':
	(.text+0x58878): undefined reference to `mfd_remove_devices'

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>