mfd: cros_ec: Add MFD_CORE dependency

CONFIG_MFD_CROS_EC requires CONFIG_MFD_CORE for a couple of functions that
are declared but not defined:

	ERROR: "mfd_remove_devices" [drivers/mfd/cros_ec.ko] undefined!
	ERROR: "mfd_add_devices" [drivers/mfd/cros_ec.ko] undefined!

Fix it by selecting CONFIG_MFD_CORE anytime CONFIG_MFD_CROS_EC is enabled.

Signed-off-by: David Rientjes <rientjes@google.com>
Acked-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>