mfd: twl4030-madc: Move driver to drivers/iio/adc

This is a driver for an A/D converter, which belongs into
drivers/iio/adc.

Signed-off-by: Sebastian Reichel <sre@debian.org>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>