The XADC has two internal ADCs. Depending on the mode it is operating in
either one or both of them are used. The device manual calls this
continuous (one ADC) and simultaneous (both ADCs) mode.

The meaning of the sequencing register for the aux channels changes
depending on the mode.

In continuous mode each bit corresponds to one of the 16 aux channels. And
the single ADC will convert them one by one in order.

In simultaneous mode the aux channels are split into two groups the first 8
channels are assigned to the first ADC and the other 8 channels to the
second ADC. The upper 8 bits of the sequencing register are unused and the
lower 8 bits control both ADCs. This means a bit needs to be set if either
the corresponding channel from the first group or the second group (or
both) are set.

Currently the driver does not have the special handling required for
simultaneous mode. Add it.
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Fixes: bdc8cda1 ("iio:adc: Add Xilinx XADC driver")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>