iio: adc: mt2701: Add Mediatek auxadc driver for mt2701.

Add Mediatek auxadc driver based on iio.
It will register a device in iio and support iio.
So thermal can read auxadc channel to sample data by iio device.
It is tested successfully on mt2701 platform.
Mt8173 and mt6577 platforms are not tested.
But the expectation is compatible.

Signed-off-by: Zhiyong Tao <zhiyong.tao@mediatek.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>