mfd: mc13xxx: Change probing details for mc13xxx devices

This removes auto-detection of which variant of mc13xxx is used because
mc34708 uses a different layout in the revision register that doesn't
allow differentiation any more.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Marc Reilly <marc@cpdesign.com.au>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>