iio: adc: meson: init channels 0,1 input muxes

Set up input channels 0,1 muxes in the same way as for the channels 2-7
later in the code.

Signed-off-by: George Stark <gnstark@sberdevices.ru>
Link: https://lore.kernel.org/r/20230715110654.6035-2-gnstark@sberdevices.ru
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>