mfd: Implicitly register mc13xxx led subdevice

A led subdevice is registered now iff the corresponding platform data is
available. Without platform data the device isn't usable so this is a
sound check.

Signed-off-by: Philippe Rétornaz <philippe.retornaz@epfl.ch>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>