iio: adc: meson: support reading from channel 7 mux inputs

Add IIO channel for all muxed inputs of channel 7.
Meson saradc channel 7 is connected to a mux that can switch input
to well-known sources like Vdd, GND and several Vdd dividers.

Signed-off-by: George Stark <GNStark@sberdevices.ru>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20230714114010.293440-7-gnstark@sberdevices.ru
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>