iio: adc: sun20i-gpadc: support non-contiguous channel lookups

Using consumer driver like iio-hwmon which resolve channels through
io-channels phandles will fail for sparse channels because IIO core by
default treats phandle argument as index into channel array.
        eg. <&gpadc 1> will fail if there is only channel@1 specified

Add .fwnode_xlate() which maps DT phandle to the registered channel
whose chan->channel matches the hardware channel number. It allows
sparse channel maps to be consumed by drivers like iio-hwmon.

Tested on Radxa Cubie A5E.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Michal Piekos <michal.piekos@mmpsystems.pl>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

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