mfd: Provide a generic version of mc13xxx adc_do_conversion

This is needed to convert the touch driver away from using struct mc13783.

Note this patch drops MC13783_ADC0_ADREFMODE. This is unused and doesn't
exist on mc13892.

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