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 and parse some DT properties
Add nau7802 to dt

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
update DT