iio: Add Nuvoton NAU7802 ADC driver

The Nuvoton NAU7802 ADC is a 24-bit 2-channels I2C ADC, with adjustable
gain and sampling rates.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
ARM: mxs: cfa10049: Switch bus i2c1 to bitbanging

The ADCs connected to this bus have been experiencing some timeout
issues when using the iMX28 i2c controller. Switching back to bitbanging
solves this.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
ARM: mxs: cfa10049: Add NAU7802 ADCs to the device tree

Since these ADCs share the same non-configurable address on the I2C bus,
they have to be put behind a muxer.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>