iio: xilinx-ams: Add labels

Label all the channels using names from the reference manual. Some of
the "control" channels are duplicates of other channels. The reference
manual describes it like:

> The AMS register set includes several measurement registers that are
> written to by the PS SYSMON unit using the single-channel mode
> (sequencer off). These voltage measurements are performed using the
> unipolar sampling circuit with a 0 to 3V range and do not have alarms
> or minimum/maximum registers.

So I think these really are measuring the same voltages but in a
different location. In which case, sharing labels makes sense to me.

Signed-off-by: Sean Anderson <sean.anderson@linux.dev>
Link: https://patch.msgid.link/20240620204842.817237-1-sean.anderson@linux.dev


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>