dt-bindings: iio: Fix adi, ltc2983 uint64-matrix schema constraints

dt-bindings: iio: Fix adi, ltc2983 uint64-matrix schema constraints

'minItems' and 'maxItems' apply at the same level as 'items' schemas as
the keywords apply to arrays. What's currently defined is a 3
dimensional matrix with the outer size being undefined. To fix this,
minItems/maxItems needs to be moved up a level.

With this fixed, the example fails validation. For matrix types, the dts
syntax must use brackets (<>) matching the schema definition. In this
case, the inner array size is 2 elements, so let's add the correct
bracketing.

Fixes: 3986a14870cb ("dt-bindings: iio: Add ltc2983 documentation")
Cc: "Nuno Sá" <nuno.sa@analog.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Hartmut Knaack <knaack.h@gmx.de>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Peter Meerwald-Stadler <pmeerw@pmeerw.net>
Cc: linux-iio@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Rob Herring <robh@kernel.org>