IIO: Ingenic JZ47xx: Add support for JZ4770 SoC ADC.

Add support for the ADC hardware present on Ingenic JZ4770 SoC.

Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Tested-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>