mfd: New mc13783 function exposing flags

This is needed for the mc13783-adc driver to decide if a touch screen is
connected.  If so some channels are not available as generic hwmon inputs.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>