dt-bindings: iio: adc: Add imx6ul & imx6sx compatibles

Both are already using the vf610 compatible.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220613123529.466528-1-alexander.stein@ew.tq-group.com


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