IIO: Ingenic JZ47xx: Add xlate cb to retrieve correct channel idx

Provide an of_xlate callback in order to retrieve the correct channel
specifier index from the IIO channels array.

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>