iio/adc: ingenic: Retrieve channels list from soc data struct

Instead of having one array of struct iio_chan_spec for all SoCs, and
have some SoCs remove the last item of the array as they can't use it,
have each SoC define its array of supported channels.

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