iio: adc: meson: add separate config for axg SoC family

According to Amlogic custom kernels ADC of axg SoC family has
vref_select and requires this setting to work nominally and thus
needs a separate config.

Fixes: 90c62418

 ("iio: adc: meson: init voltage control bits")
Signed-off-by: George Stark <gnstark@salutedevices.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20231127235558.71995-1-gnstark@salutedevices.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>