mfd: twl-core: Register twl4030-madc child only for twl4030 class

twl4030-madc driver can only handle twl4030 class MADC. The newer revisions
of twl does not have MADC, instead they have different IP called GPADC which
is not backward compatible.

Acked-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>