mfd: cros_ec: Export needed symbols for the i2c and spi modules

In order to fix those, when all cros code is modular:

ERROR: "cros_ec_prepare_tx" [drivers/mfd/cros_ec_spi.ko] undefined!
ERROR: "cros_ec_register" [drivers/mfd/cros_ec_spi.ko] undefined!
ERROR: "cros_ec_remove" [drivers/mfd/cros_ec_spi.ko] undefined!
ERROR: "cros_ec_suspend" [drivers/mfd/cros_ec_spi.ko] undefined!
ERROR: "cros_ec_resume" [drivers/mfd/cros_ec_spi.ko] undefined!
ERROR: "cros_ec_register" [drivers/mfd/cros_ec_i2c.ko] undefined!
ERROR: "cros_ec_remove" [drivers/mfd/cros_ec_i2c.ko] undefined!
ERROR: "cros_ec_suspend" [drivers/mfd/cros_ec_i2c.ko] undefined!
ERROR: "cros_ec_resume" [drivers/mfd/cros_ec_i2c.ko] undefined!

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>