iio: adc: mxs-lradc: Add support for ADC driver

Add support for sixteen-channel 12-bit resolution ADC and its functions,
which include general-purpose ADC readings, battery voltage measurement,
and die temperature measurement.

Signed-off-by: Ksenija Stanojevic <ksenija.stanojevic@gmail.com>
Reviewed-by: Jonathan Cameron <jic23@kernel.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>