Add NAU7802 ADC driver

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Add nau7802 to dt

Use i2c_gpio for the i2c as we get a lot of timeouts when using i2c-mxs.

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